Much of the advice in this thread is either “Do xyz and you’ll have better chances!” or “It’s ok to be unattractive, it doesn’t mean you’re bad! Feel better!”. But that doesn’t answer your question.
It hurts, a lot, to not have intimacy for extended periods. It can burn, it can ache, and it can be a slow, subtle sort of pain. It can give rise to bitterness, as you call out, and to anger, sadness, listlessness, frustration.
Why don’t you want to be bitter? It’s a painful thing that is happening to you. That feeling isn’t wrong, it’s telling you something. If you feel a lot of pain and then suddenly stop feeling pain, that is very bad - it usually means you’re dying or your nerves are damaged. There’s no quick fix or silver bullet that will allow you to hold this like an old stoic, it’s just a lot of work.
There’s three places you can intervene: thoughts, words, and actions.
Thoughts are where this starts. If you don’t have bitter thoughts you won’t have bitter speech or actions. When you have bitter thoughts, just let them be. Don’t spiral - feeling bitter about intimacy isn’t great, but feeling bad about feeling bitter strengthens both, and it feeds itself from there. When you notice yourself spiraling or wallowing, just stop. Find a distraction or will yourself better or whatever, just don’t let it feed itself. Meditation might help if this mental action is difficult. If you can find a positive channel for these emotions (which is quite hard to find), use it!
Speech is the first layer where this can affect others, but it’s significantly lower stakes than actions. Generally, be conscientious. People can’t willingly un-know things so be careful with what you share. Don’t vent unless someone willingly signs up for it (which you can ask friends to do!). Don’t put this out like it’s a problem for someone else to fix or the worst thing that ever happened. It sucks, but it is manageable - you are managing it. When you make mistakes, point and call them. Say out loud “I did/said xyz, that was a mistake because abc, sorry, next time I’ll do/say mno instead.” This helps make a memory for you and others so you actually fix things and opens the door for feedback.
Actions will hopefully only come into this positively. Do the things that make good thoughts and speech easier. Learn to recognize how frustration and anger and bitterness feel in your body so that you can better notice+control them in the moment, and so that you can physically release that tension/sensation. If you feel urges to hurt others or yourself seek therapy.
It sucks. It hurts a lot in ways that many don’t understand or sympathize with. And it is itself a significant barrier to intimacy. I haven’t figured it all out myself, but I hope this helps. Good luck.