How on earth can I trust my partner, when its my fault i dont.?

I think that feeling is normal. You just got to tone it down. LOL We all feel the greem monster when our loved one has shared the past with someone. It is part of maturing. I guess. SO I am told. LOL LOOK dear, what you have to do is set yourself straight. Ask yourself if you want to be with him. If you say yes, then know you made a decision that leads to taking actions. Take actions towards your decision. You have to commit to what you decide. Until you can't no more. No inbetween stuff. This means you have to trust him if he hasn't done anything to make you not trust him. If it is too overwhelming for you then you need to accept it and consider making a change to your relationship status at this point.