I can sympathize with how you are feeling. I have been eping since my son was one week old; and he will be 5 months old next week. I have never been more than an LVP, on my best day ever I got 23 oz. At 4 pumps a day I was getting 15 oz and had made peace with that.
I am insulin resistant and have been advised by my doctor that I MUST lose weight in order to prevent developing diabetes. A week after starting weight watchers AF came and my milk production dropped to 8 oz a day.
I added 10 minutes to each pumping session, and I'm back to about 11-12 oz but I wonder too when to give up. I don't feel emotionally ready yet to stop, so I'll keep going. I have always had to supplement with formula. My original goal was to make it to 12 weeks, then to solids, and then to six months. I've completed the first two, it's only five more weeks to six months and then I will reevaluate again.
I think the best advice I can give is the same as what many others say here: Never quit on a bad day. If you make the decision not to pump anymore, make it deliberately and feeling good about the BM you have provided and not mourning the milk that you were unable to provide.