I sail an FT158 with a Naish Stealth 9.6 @ 80-85kg.
Try lifting the toes of your front foot to hook the strap to keep from popping out. If you're seriously imbalanced (being twisted to the rear) check your harness line balance (shift to the rear?) and/or sail draft (too over-powered?). Dropping the boom 1-2cm also helps to give more front foot pressure.
For upwind: pushing over the top of the fin shouldn't make sheeting-in more difficult, but easier as you can straighten the leg to get more power. Make sure you are rolling the lee rail down some to get the fin-tip tracking up wind.
Is the sail raked back enough (closing the slot - with a "race" sail)?
Try twisting the heel of your rear foot forward in the strap (I set the strap with the rear screw in the rear most plug, and the front screw in the middle plug - I wear booties all year and this setting allows me to pivot my foot in the strap to shift balance fore-aft), and swing your weight in the harness forward - you'll read "trying to look round in front of the mast" - keep the rig raked back! Keep the fin pressure on, and hook/scissor the front foot to windward.
Try and relax your upper body as much as possible; don't over-sheet and stall the rig (lose power); and, if not over powered, keep any weight in your toes of the back foot.
Hope some of this helps - Martin