Why can't I just let go...

Things are going good really. But for some reason I just can't let go and totally enjoy it. It's like I just keep waiting for something to go wrong. And that is not something I have ever done when it comes to our relationship. But I do now. Even though he is treating me the way he did before the affair...it's just not the same. Maybe it's because everything has lost its innocence. I am not saying I don't think he doesn't love me or anything like that. But I don't want to be naive and assume he loves me as much. Because I don't see how he can after disreguarding me to have an affair. It's just so hard to let it go. I want to...but I am afraid. I am afraid to love him as freely as before...to believe everything he says...to totally trust him...to give all I have to give. Because when I did that before he had an affair and shattered my heart. I don't want to be blindsided again. So I ask myself how does one fully recover from an affair...whether it is with the person who committed the affair or with someone new? How do you trust someone enough to believe they will never betray you like that? It's like once it happens you are waiting for it to happen again. Like before it happened you never thought twice about it...but now it's always lurking in the back of your mind. This ugly thought that once in awhile consumes you. Takes your breath away. Takes your zest for life away. And you can push it back as many times as you want...but it keeps coming back. How do you get rid of it? How do you lock it away so that you don't have to think it anymore? How do I explain this to him. How do I explain to him I am scared without seeming too needy or desperate? How do explain to him I am afraid he doesn't love me the way he did when he married me? How do I explain to him no matter how hard I try I can't stop seeing him with her? How do I explain to him that I feel like I am his second choice...that he only came back to me because she pushed him away? How do I make the "what ifs" go away? What if she hadn't pushed him away? What if I hadn't figured it out? What if he does it again? What if he realizes all the sudden he wants to be with her after all? What if I am not strong enough to let go? What if...what if...what if! Two words that I hate! I don't want to push him away...I want to be married to him. I want our relationship to recover....but how do I let go?