Self doubt(help!!), How do i stop this self doubt that hinders my relationships?? Options

[Unrequited_love]

OK this has been driving me nuts for the last couple days and I wanted to know if you guys have any suggestions or advice on how to stop this. You see Im in an amazing relationship with a really great guy...hes never done anything to make me feel down about my self or make me doubt his commitment but no matter what i cant seem to shake this feeling of self doubt. Ive put myself into a place now that i dont see why he would want to be with me so he must be lieing and hiding something and its not fair to him or myself because now im sitting in a different city freaking out thinking that something must be going on in my home town because im not there and i know hes a better guy then that he would never do anything like that ever! its as if i believe because ive been hurt in the past that is destined to happen in this relationship and thats making it so that im pretty much sitting here and waiting untill it does happen so that i can just get it over with but yet when i sit back i can not see one reason that i can see it ending. the worst part is i know that im doing this and yet i dont know how to help and it affects him as well because if my mind goes into self doubt mode while im with him and i get all quite and upset then he starts feeling like shit to because he knows im upset. I know that my mind being in this state will just end up ruining my relationship so what do i do about it so that it doesnt? The last thing i want is to loose him.

[Unrequited_love]

Ya i geuss I know in my heart that he wont do anything but i also know that because it of my past its really hard to keep that thought in my head.

heres one reason that its easy for me to self doubt:
you see there was this girl that he dated for a year and a half so oviously shes a big part of his life and i would never try to take her away from him. and hes told me over and over that he cares more for me then he does her. But the thing is he also knows that she still likes him because shes told him over and over as well as tried to pursue him while he was dating me. and so now everytime that i hear about them hanging out or anything i become really shaky and almost weak but i dont know what to do about it because it wouldnt be fair at all for me to tell him to not hang out with her but i hate the feeling i get everytime that i hear about her. How do i get past this? should i go and talk to her? should i try my hardest to ignore it? what should i do??

like oviously if they were together for a year and a half there has to be something there!