I'm only 17, so don't take this advice too heavily since i've never been engaged. However, I think, first of all, you should attempt talking to him. No matter what, talking has to be the first thing you do. If there's still no change in his behavior, try getting him to spend more time with you, maybe do a few things interesting and different. The reason he may not pay much attention to you is that he's gotten so used to your mannerisms and how you are. If you plan something unexpected, or do something different in bed (if you two have sex before marriage), then maybe that will rekindle his feelings for you and make him realize why he's engaged to you in the first place.
Just take things slow. You have all the time in the world to either make things right (which i'm sure you want to), or move on.
