I'm grumpy because my lover is seeing his other girlfriend and I'm working and I'm mostly okay and he needs to see her too and it does me good too I take the time to reflect but I'm grumpy and I hate it. Also scared he'll decide my grumpy isn't worth it.
Being in a bad mood sucks, and you're right to recognize that it often sucks for the people around you as well. But there are things you can do about it! Learning to manage your moods is a powerful skill and there are many ways to tackle it.
It can help to just reframe things: it's okay to be moody about having to work when other people are having fun, but that doesn't really have anything to do with polyamory or what your lover is doing. Even if he wasn't seeing his other partner, you'd still have to work. So try to separate those frustrations. And if your work consistently makes you miserable, consider finding another job, if possible.
If you're worried that your grumpiness is going to make your lover decide that the relationship isn't worth it, try finding other outlets for your moods so you can bring him more positivity. Anything from online support forums and chats to meeting up with a friend to professional therapy can help with that.
Finally, remember that grumpiness can sometimes be banished using positivity strategies. You may have heard that you should "vent" your negative feelings, but in fact, letting yourself mull and stew and ruminate will only make things worse. Try and refocus and distract yourself. Put on peppy, upbeat music and bop around. Read or do something that makes you happy. Have lunch with a coworker and don't talk about the stuff that's bugging you.