How can I learn to trust him ?

My boyfriend has lied in the past and given me broken promises. Sometimes I just don't trust him. Nothing he says. I feel bad that I feel this way because he does a lot to try and prove that he's a good man, but I often wonder if his intentions with me are good or he's just stringing me along until someone else comes along. I don't know if it's my trust issues or his lying issues that has made me feel this way. I feel like I come second and I'm not a priority in his life. I feel bad because he tries to tell me that I am but I don't believe him. I don't know how to talk to him about it and let him know how I feel without it turning messy. What should I do ?