No good deed goes unpunished OP.
And as this thread shows, PTAs can be very divisive.
In my experience, it often ends up with a ridiculous burden being placed on the few, with consequential feelings of resentment (acknowledged or not) as others won't get involved because of the expectation of so much work. Said PTA then perceived as clique-y by other parents who would be interested in helping a bit and those who simply can't be asked.
Don't feel bad, whatever you decide. It really shouldn't just be down to you.
Ask for a co-chair or an agreement that others will share the burden in a clear and tangible way, if you want. Or say you will just do it for one term. Whatever would work for you, and stick to it. But don't be brow-beaten into something that will stress you out, or that you don't want to do - just because some people would rather you shouldered the burden rather than them.
Would agreeing to do a term (with limited events) be an option? Making the events conditional on getting enough firm commitments of help for specific tasks? Say you'll stay on for a term if you get class reps from each year group to share the load?
For my sins, having been on an imploding committee (which would have given the Borgias a run for their money), I resigned and said I would be happy to help out on events for a set period of time - otherwise you end up never getting to spend any time with your own child/partner at the various events you are meant to organise. Mostly, the (self) chosen few however have strong views and prefer to struggle on themselves.
Sorry if that sounds a bit bitter, but the scars of the PTA can run deep.
Tempted to say run for the hills. But PTAs can and do a lot of good. And you are probably in this situation because you are "too nice" and didn't want to say no. But it really should not be down to you.
At the end of the day, if it is leaving you stressed out, don't do it. If you can't make it work for you, resign before it snowballs. Your first duty is to yourself and your family. No way is it worth losing sleep over.
It may sound really weird, but I have seen far more acrimony, in-fighting and spite on voluntary committees than during my years in the City!