The minute you start to care for someone or something, there is pain at the end of it.
That is a simple fact of life, but it is very very hard to live with - especially if you have been hurt before.
In order to fully commit to a relationship, you have to be vulnerable, the two go together, that is what trust is all about.
You can only ever put yourself into your relationship, and see how it goes, there are no cast iron guarentees that anyone will not hurt you again, but you cannot allow that fear to prevent you from the chance to love, and be loved again.
Remember, although your past experience was bad, it will have been a strenthening time, even if you didn't feeel it then, and you don't feel it now.
Your past experience means that you know how bad a breakup can be, and you also know that you do live through it, and you do find yourself ready to love again.
Try and avoid thinking of your relationship in terms of its potential ending, and the pain that will cause, and enjoy your time now, let the future take care of itself. Too much time spent dwelling on a potential future disaster will poison your present, and could lead to a self-fulfilling prophesy.
Have a frank talk to your partner about how you feel - if he is right for you, he will understand, and you will know, and be able to move forward with more confidence, which you can both build on.
Reticence based on experience is human nature, but remember, this man is not your previous partner, don't make him pay for the hurt caused by someone else.
Good luck.