I may be in the same boat. I know I have had commitment phobia, and it seems to keep coming up after a few months (or sooner). I also have a general anxiety disorder according to my shrink. It seems in my experience I hadn't found the right type of person who could understand my anxiety. I am with a truly amazing mono guy now who is not threatened at all by my desire to be with other people.
It is good to find the root of your relationship phobia. For me it was partly the fear that the person might not be the right one, partly because I felt incapable of breaking up with anyone (fear of hurting anyone) and particularly the guy I was with whom I really care about and could never stand hurting (I still love him but I had to break up with him for the final reason which he could not accept
also because I am pretty sure I am poly.
Having successfully broken up with him and had a GOOD breakup (really the best as far as breakups go, we are good friends who hang out and joke a lot), realising I have the strength to make my own decisions and am not with someone who will not fully meet my emotional needs and yet will not let me find that elsewhere, and realising that I now have the freedom to be poly (all in my current relationship) has gone a long way towards alleviating my commitment phobia.
If you find out what is causing your anxiety it will help a lot. Being able to relax and not overthink or freak out, and not condemn yourself for your feelings definitely helps with clear thinking. Honesty and openness is very good provided it is a healthy relationship (healthy as in not one where the other person is very manipulative or an emotional abuser) and assertiveness is a must: deciding what you are willing to compromise on and what you cannot under any circumstances compromise on. (For me my poly-orientation as nobody can tell me who I may love.).
Commitment phobia really can be treated. I'm pretty sure it's related to anxiety. Some good therapy and learning what structures you particularly need to put in place to feel in control really makes a difference (eg reserving the right to take some "space" or being sure that you are free to do what makes you feel secure that you are not losing your identity, or whatever your personal factors are).
I am sure with the right focus, confidence in yourself and the right support structures or necessary counselling you really can get through your commitment phobia. And you don't have to justify being poly even if you do see it as a result of your commitment phobia. You have the right to be who you are.