I find I tire really easily when i use bigger sails..6.6m / 5.8m ( weigh roughly 64kgs)..I go ok for a while but then find it really hard to pull the sail in with my body.- especially going upwind.Not sure if it's just me with lack of fitness & age or do all us ladies find sails about that size compared to your weight hard work? 
..Have you had a sex change?
Weight training really helps; just back after 8/9 mnths off; wasn't the shock to the body I thought it would be
You sound overpowered. Have you tried switching down to a smaller sail? 
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Try fine tuning your harness line positions. I have found that sometimes the smallest changes for the same size sail can make a difference to endurance. The Same setting does not always work for different wind strength, board size, mast position, wave angle etc. Also look at slight changes to boom height positions for the same reasons. Usually we tend to just set these things the same way every time.
Sorry not a girl but I struggle to have the same endurance I had 30 years back as well, mind is willing but body is slowing...
Probably a combination of old age and fitness Can't do much about the old age, but i find swimming laps is good for fitness, you can combine easy laps with sprints, using a pool bouy isolates your legs and makes you work the shoulders more, kicking laps (with or without fins works the legs), doing a butterfly kick on your back (with fins) works the abs and legs.
I don't get to spend much tow these days but if the fitness is kept up its not to bad when I get out and really hard if i don't keep up the fitness.
Old age.... in my twenties i had less than half of my current lung function and was a lot fitter than now. spent more time on water and could sail all day with out any problems, now days I'm lucky to last a couple of hours when i can get out.
The moral of the story is , build up your fitness and try to maintain it and it will be easier to sail.
sorry to interrupt the "girls" section but it's definitely alot if not all about tuning,
top riders will have their rigs really finely tuned mainly to go faster but this tuning is important because it makes everything easy to use too no matter what the sail size
but then again dropping your rig is definitely more tiring than straight sailing, never the less there are still tips and tricks to make life easier on these fronts too. you just have to look around at what others do and ask questions
It would be nice if you could get the answer from the girls at your weight and height, but until they front up, I would suggest that setup would help a lot.
I have tried setting the sail for the lightest wind, but I came to the conclusion that I was better setting the sail up to handle as well as possible instead. That generally means having enough downhaul and outhaul to keep the centre of effort as stable as possible. A sail set like that feels light and easy to handle. Maybe loses out slightly on ultimate bottom or top end, but it just feels a lot more comfortable to sail. To get that, you need the sail and mast to be well matched.
And at your weight, a 95 litre board can be easy to uphaul, but its a lot less work to waterstart it if there is sufficient wind. You probably use a lowish boom anyway, so it could work to set the mastbase position to allow you to pull the boom onto the tail of the board for waterstarting, which can save some energy in a very light wind.
I try to sail with minimum effort, so these are things I have found will make sailing less strenuous.
Get your harness lines balanced before you go on the water. The back strap 1/3 of the way along is a good rule of thumb but I stand my sail up on the beach and move the lines until I can hold it in balance just holding the loop of the harness line. Go for a run out on the water and if you feel pull in one hand more than the other, adjust that end of the harness line. You want to aim for the situation where you could sail no hands if you were brave enough. Then adjust the boom height or harness line length so your arms are straight. Then sail across the wind and try for even pressure through both feet, with your hips parallel to the board. If you don't feel properly balanced try moving the mast base forward or back in 1 cm increments until it feels right. I'm presuming you have a fin which matches your sail...
Then off you go...
When you feel you have got it right jot down the harness line position, mast base position, boom height, wind strength and sail size and you will have a good guide for the next time you use that combo.
My 5.8 is my fave sail and I'm very comfy with the 6.4 also. The smaller sails are very light and fun to use but tend to be used in more trying conditions so end up being more tiring.
Not female, but 67kg so can hopefully can add something.
I have felt a lot more comfortable since getting adjustable harness lines and tend to tweak the length for each session. You can adjust on the run. A boom with lots of markings also makes it easier to get line position right as did bringing the lines closer together - that took some getting used to initially.
Waterstarting can really sap the energy. My wife is 54kg and found a buoyancy vest makes all the difference.
Spend some time practicing uphauling your smaller boards. I can now uphaul 78L at a push although not if too choppy.
Luckily in those winds I don't stuff up too often but when I do I tend to uphaul if I can .The mast generally ends up pointing to the bottom of the lake & I have to kick it up with my feet get the mast tip flying & walk up to the boom to start..
I don't want to waste my sailing energy but i suppose I'll have to for long term gain.
back.
If you are having to push the boom away with your front hand, the harness lines are too far forward.
and when going downwind you should sheet out a bit, and swing your body back so you are more over the tail of the board. You don't need to sheet in, only balance the sail. Let it fly.
I'm a girl and your weight - but not sure that qualifies me to answer!
I don't get to sail nearly as often as I would like so find that fitness is definitely an issue. However, I don't find or think it should be linked to sail size or struggling to hold a sail down. Assuming wind strength stays the same, holding a sail down is primarily about your weight and commitment to the harness. This should not change as you tire.
A few things that spring to mind...
1) As tough as we think we are, we simply don't have the muscles of windsurfing dudes. If you consistently feel uncomfortable, are you applying enough downhaul? Not uncommon I think for women to not have enough. (Expressions of interest for lovely man to follow me round and tighten my downhaul are welcome ).
2) Is your boom high enough? Without making yourself over powered, a low boom can make you feel like you're struggling to hold down the sail when you're light and girly.
3) As for many responses above - play with line length and position. I think you've mentioned before that you use a seat harness? If you're uncomfortable, maybe try a waisty for something different. I know it's a bit 'each to their own', but I find a waisty helps me to hold down a sail much more comfortably. Also, lines further back helps heaps for early planing and keeping the rig more upright, but I find it does make it harder/more uncomfortable to go upwind.
Fitness is certainly a big issue for me and limits my sailing. Mostly my arms. As the session gets longer, I find it harder to trim/adjust the sail and therefore more uncomfortable. Also harder to gybe. As soon as I hook out and my arms have to do some work, I fall apart.
Hope all these responses help! Windsurfing - the one place chicks can quite comfortably talk about their weight!
I think you use way more energy up hauling , especially in the chop of Lake Macquarie , water starts are way easier !!!!!!
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) but compared to the guys I'm a lightweight!
