I feel your pain. I broke up with the last girl I was seeing over being poly. I'm 26 and I feel like it's hard being in your twenties and making realizations that most people don't until they're in their 30s or 40s. The pool of people who have a very high level of self discovery or awareness is low and the number of people who find it early in their lives even lower. I recognize the fear that I have in finding someone has some of the same awareness I do.
I'm going to come at this at a little higher level of abstraction (broader terms).
I think that suffering comes from the thoughts that we have about what will happen and what is happening, not from the event itself happening (hope that makes sense). We especially suffer when we believe these thoughts. It's like bungee jumping - you're nervous and scared before you jump, but once you do you are completely in the moment, completely present with what is happening to you. When you're falling you don't have time to suffer! Our thoughts are what are make us suffer, what can prevent us from jumping off the edge of uncertainty.
If I was to point at the root of your suffering, it'd be this:
I don't want to start over again. I don't want to find someone new. I don't want to go into this dizzying world and have 90% of the people I'm interested in reject me because of the way I love. I'm very scared of being alone....I haven't been single for any significant amount of time since I've been 15, and the idea that I may be alone for a long time is terrifying.... I'm a very needy person. Honestly, I sometimes think that keeping me happy is simply NOT a one-person job.
What I hear here is that you're saying that "if I break up, I won't find someone who will accept me for being poly." and "I need to be with someone to be happy."
I would look at those thoughts in yourself with bold honesty to discover if they're true.
Are they true? If you break up, is it possible that you could find someone/several people to be poly with and be happy with in the future? Could you find a poly group in your city? Could you date someone new?
Do you really need to be in a relationship with someone to be happy? Could you spend more time with some amazing friends and use them to fulfill the desire to be connected with others? Could you use the time that would spend in a relationship to build your success in other areas of your life (career/family/etc) and find happiness in doing those things?
There's an entire world of possibility laying in front of you. If your truth is that you care about two people and want to be fully (physically/sexually/emotionally) with both of them and that's not possible, than the path of least suffering is to accept what is, make the best possible choice, and move on to the next stage in your life. The only real difference between being nervous and excited is perception. Personally I think you're at one of the most exciting times in your life!
If what I'm saying resonates with you, look at these couple of videos by Byron Katie:
I need a boyfriend
Christine who Wants her Boyfriend to be Monogamous
Her Steps: The Four Questions and Turnaround
1. Is it true?
2. Can you absolutely know that it’s true?
3. How do you react, what happens, when you believe that thought?
4. Who would you be without the thought?