Looks like the bottom line here is that you are being overworked because reasons.
I submit that "reasons" should not really matter to you. There will always be reasons - lazy coworker, new customer onboarding, big customer coming but only if we have a new feature X, other initiatives from sales, marketing etc etc.
Whatever the reason, the fact is that you get more work on top of your existing work. Nothing unusual with that.
Not much you can do but accept it. What you can (and for your own sanity, must) do is that, when you get more work X from your boss, spend some time with him/her to go over your timelines and adjust them. This is also a good time to ask which has higher priority - should you do X first, or do it when you are done with your existing stuff?
This often works. Even the most Dilbert-y PHB will likely accept that you need more time to do more work.
If not, well, I guess it's time to re-evaluate the situation.