At one time or another everyone has been burned. They have been betrayed and then they enter new relationships. Sometimes the make the mistake of bringing their old baggage into their new relationship.
One thing you have to know is that all guys are different. Just because someone in the past hurt you does not mean that every guy you date will. There actually are good guys out there, they are just harder to find.
If you were hurt in the past make a list. Compare the guys in the past and what they did to you to the guy you are dating now. You will see the difference. If the guy you are with now has more pros and did more for you than the guys in the past did than you will see that you have a good guy.
Ask yourself is there any reason why I shouldn't trust the guy I am with? If you can't trust him than you shouldn't be with him.
Don't talk about your past. You don't want to scare the guy away because it could be stressful for a guy to always reassure a girl how much he cares for her. He wants the girl to know that she means a lot to him.
Give it time. If you have been betrayed many times in the past it is normal to over think things. With time you will see weather or not the guy is the one for you.
Love is a great thing and often times people's trust and insecurities ruin it. If you have a good guy don't let anything get in the way.