I'm one of those people that took me three or four months to learn to do a roll. I had such a mental block. The key as others have said is practice, but also don't be afraid to go at your own pace. There is nothing wrong with having to take some time to learn to roll. My problem is that I had about four of five people who had about 10 different ways each to tell me to do the same thing. I just learned to block them out and worked myself through it on my own.
My biggest problem is that I thought about what I was doing too much. I would sometimes fixate on a point then I would freak out. I just learned to do the rolls without thinking. Trust your body, and you can do it.
I hope that helps,