Taking a new bit to an open mic and watching it die in front of your eyes is brutal and the urge to never say it again is tempting, but you should resist it. You can’t make a decision 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 come up with a new tag or 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 and if you keep working it out, that joke is going to 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 that you haven’t quite 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 the sudden you have 5 new 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.