How can you learn to trust someone?

I have been married for almost 4 years and I have horrible trust issues. The sad part about the situation is my husband has never done anything to me for me to not trust him. I hate that I am suspicious of things that he does, when deep down I know he has done nothing wrong. I am so afraid of him hurting me that it's like I am preparing myself for it. It's that stupid? Because I am afraid of losing him, I push him away with all this false allegations? What is wrong with me? I love him and I want this marriage to work. He tries so hard to prove to me that I can trust him and that he loves me, but it's never enough. Am I doomed to destroy this relationship over nothing? How can I learn to fully trust him? It scares me because by giving him all my trust and havin faith in him that he won't betray me means I could get hurt a lot worse than if I thought I could see it coming. Again, it's like preparing myself, when in truth I am just probably going to cause it. I need to trust him and stop pushing him away. Please give me advice. Also, just because it may be relevant or you are/were in a similar situation, I am bipolar. I am on medication now. Could all this paranoia be caused by this? Will the medication make it go away?