There might be some things in your past that you are embarrassed about, or you know might hurt them, so you might have a little problem saying it. On the other hand your relationship should be at a point where you should be able to tell them anything.
The thing is to know your partner well enough to know when and what to tell them. Don't tell them everything all at once. Take time to tell them things, a little at a time.
What you could do is tell them up front that there are things about your past that might be a problem, allow them to choose to know or not.