Taking a new bit to an open mic and watching it die in front of your eyes is brutal, and the urge to never repeat it is tempting, but you should resist it. You can't decide about the quality of a joke based on one set alone- you have to run the experiment of that new joke a few times so you can track the results. Most of the time, it is during that 2nd or 3rd run-through that I find myself riffing in a direction I hadn't thought of the first time around, and suddenly the joke becomes funnier. Don't be afraid of a joke being weak at first because open mics are like going to the gym. If you keep working it out, that joke will tone up and start looking ripped as hell onstage- that's what they're talking about when they say call people "a strong writer."
If something still isn't making progress after rewrites... just put it away for a while. Keep a document or notebook of stuff you haven't figured out yet and revisit it from time to time. Think of these jokes as spare parts, like that bag of screws you've moved to 3 different apartments: You could throw them out, but you never know when they might come in handy.
I guarantee you that at some point, you will be writing a new bit and suddenly find a way to connect that joke that never worked to something that does, and all of a sudden, you have 5 fresh minutes.
Good writing takes time, and the sooner you accept that you're not going to write anything groundbreaking on the first try, the sooner you will write things that are.