Doll,
Is it because of another reason too? As the relationship progresses, you put in more emotions, thus you get fearful of getting hurt?
I think our past experiences will affect us somehow. Remember that there's no way to control everything in life. In the past, I was ambitious and wanted everything in my control. So when things weren't my control, I got worried and fustrated... Now I'm happy-go-lucky instead.
Your BF needs to help you by giving you more reassurances.