It could very well be that he felt smothered and didn't have enough space or didn't feel like he was in control, of his future. It could also be that you are pressuring him too much now and all he wants is to be away from you. Hopefully this is the case but if you push too much it very well could be. If what you want is you and your boyfriend get back together you have to be aware of this.
Your boyfriend may be like many males and thrive on being independent. They need their space from time to time. While the idea of marriage and commitment may have seemed appealing to them for a moment, reality could have set in and now they are feeling trapped. Males very often need to feel in charge. They need to feel in charge of not only themselves, but of situations.
If things start getting fast they don't feel so in control anymore. Once marriage or long term commitment starts getting discussed it is easy to feel that you are getting sucked into something if you don't feel ready. If they feel they aren't ready, they most likely aren't. If they feel like they are getting pushed down a road they don't think they want to go yet, they may push themselves away.
If he has pushed himself away and you really want you and your boyfriend get back together you are likely going to have to be secretive and sensitive about it. You will have to decide that you don't want to force him down the road. You want him to just walk with you and let him feel he is taking the lead. He needs to feel like he is choosing this direction.
Since there is no direction right now and you are dealing with a break up, now is actually a good time to get started back down that path. He may feel free and clear of you and to an extent very well may be. If he has any desire to talk with you and you are still on a friendly basis, just be available, but not too available. If he is going to call you, make it seem like you are hard to get on the phone. If he wants to make some plans with you, turn him down at first but ask for a rain check. The goal is to make him seem like he is pursuing you. Let him be in the hunt.
Also let him have something to hunt for. Sometimes the things most desired are the things that are hard to get. Those will also be the things that makes them more likely to go in for the kill which is what you want him to do. You want him to commit and if you make yourself someone worth hunting and let yourself be hunted and not necessarily caught, you may see you and your boyfriend get back together.