You won't be a 22.4 forever. When you become a RPCD, you'll be able to get on the 9.5 list. They'll have to reduce your workload. You'll still have some really heavy days but only sometimes.
If you can think of a better plan for your career, I say go for it and leave UPS. If not, then it makes more sense to stick it out. Going back to part time isn't really an option unless you quit and get rehired. I believe it is possible but incredibly rare. Check with your steward/BA but I give you a 0% chance of that happening for you.
One last thing. You say you're a very hard worker, and that's great. But don't let them take advantage of you. Here, they like to take work off the slow guys and give it to the fast guys. If you do 200 stops and get done at 4pm, they'll look at it like, oh,
@Itchris needs more work and they'll give you 250 stops. So my advice is work at a pace that you can sustain for years to come without killing your body. Work safely, use the handtruck. It will take longer but that's not your problem.