Hi Sara,
Ok, here goes, keep in mind I could be wrong here but here is my best guess as to what is happening, it could be one of two things.
1. The relationship is based on physical, chemical attraction. If this is the case then the attraction between you is like a drug and the longer you are apart the weaker the attraction becomes, just like the withdrawal symptoms of a drug. The chemicals are waring off.
If this is the case then there is not much of a relationship anyway, it probably couldn't last. These types of relationship are hot and passionate but they are built on sand, eventually it will collapse. Not much you can do here. The other possibility is this.
2. The relationship is based on more than physical attraction, it is built on rock, that being solid communication, mutual interests and support etc. If this is the case then I would imagine that at least at a sub conscious level you know that due to the visa situation you are going to have to marry him, probably sooner rather than later if you are to be together.
If you are not ready for this, again at a sub conscious level you will start to shut down and sabotage the relationship in order to avoid this outcome. This could be why you feel the way you feel.
The solution to this if you wish to be with him is to face the situation and deal with your feelings about it. The reality is that you are probably going to have to marry him, if you accept this and embrace it that probably your feeling will return. If that is not a possibility for you, then sadly I think you know what needs to happen.