What I would suggest, is doing blind/blank bale practice, concentrate on form for a few weeks, this will also help you use those muscles and get used to engaging them as needed in shooting, as said above , a lighter bow would come in handy for this but since that's out of the question for now, the blind/blank bale is the next best thing. You could also look into making or buying a form master which will help.
Just to make feel a little better, my older son is about 5'7 @ 145-50 lbs, he swings two 70# kettle bells and usually works out with a 106 pounder,he's a solid brick wall, and I can hold my bows back longer and more steady then he can, so don't let it get you down because once you get the hang of it, you'll do just fine !
P.S. I don't know what the average hold time is but, I don't think I hold on target for more than 2-3 seconds but, I have been shooting a long time.