I just entered a new relationship, about a month and a half ago. I have been single for the longest time, and had convinced myself I would be single forever. Any negative beliefs and thoughts about relationships, had barley come through my mind, and I didn't indulge in it, because there were no external circumstances to trigger these things.
That was up until my boyfriend came into the picture. As soon as we started our relationship, all these negative thoughts, beliefs and traumas finally came to the surface. He's just as paranoid, just an FYI. Anyway, so I would constantly ask him for reassurance, and judge myself for feeling this way/being this way. I would resist these thoughts, thus the feelings, by constantly trying to find things to relieve myself of these thoughts. It was until I remembered the happiest days of my life: when I was an observer of my mind, and in control of it; rather than letting it control me. But if you were to ask how I am now? I react to my mind, identify with all these thoughts.
My relationship has triggered this growth and has reminded me I need to get back to just simply observing my mind again, and dis-identifying from these thoughts, and being in control again. So far I have allowed myself to mourn my old thought processes, and traumas, in order to replace it with more beneficial and positive thought patterns. I would allow thoughts like fears of cheating, abandonment, past hurts to come up, without judging or attempting to find a way to "feel better" or find external validation this thought isn't true. It's not true, if you don't indulge or add a positive charge to it. (I know the truth about the law of assumption, and law of attraction). As a result of all this, I am currently experiencing a mental/emotional detox/release from all the pain. I am finally letting go, and while it's painful--it's better to let it out, otherwise it would stay within me. Our relationship has been much better, and not only that--I am finally finding peace within myself!
After being single for quite a long time and thinking in the same way as you did, I can just confirm you are not the only person facing this problem. It is much harder than I thought and I need a lot of effort and enegy to combat this problem. It is a complex situation where both sides have to be willing to build up thrust and work on it. Good luck, dear!