Cold feet or something worse?

I've been with my boyfriend for almost 3 years, and we've been engaged since last December. I used to think and feel as though he were the one, but I haven't felt that way since before I accepted his proposal and now more than ever, I don't feel any happiness in this relationship but am too scared to hurt him by breaking it off. I've always wanted to be married and have kids and thought I could settle just to obtain those things, but now I don't feel as though I can settle. But I also fear that I will never be with someone who will make me as happy as I know I can be. There was someone from my past that fit this position, however we've never had the correct relationship timing and now I may have lost the opportunity to ever experience what kind of happiness he could have brought me. I don't know what to do and I'm extremely depressed now. Help.