I love coming here to get an update on my brother's (Ginnel, who I live with) relationship, I was wondering what his cryptic "its going alright" was about last night.
I have no easy answer for the doubt that springs in, I'm of the opinion to enjoy it while it lasts, if it is not enjoyable talk to her about it, don't keep it to yourself, that's the most destructive thing that can happen I think. I've been in a place where I stayed in a relationship because I just kept in mind the good times I had at the beginning without addressing the fact that I wasn't feeling much for the other person at that time, when it comes to light it's not pretty.
If possible don't try to fret about it, ask yourself what could make it better, maybe you could not see each other as often, absence does make the heart grow fonder, maybe there is nothing you can do, maybe it's just a temporary blip, perhaps you just want a bit of space, but if you've talked about it and she's understanding that it is not all happiness and roses from your side then well that's being grown up, be honest tell her what your feelings are, then it won't be as bad if anything does happen.
I've had the same doubt about not being able to love anyone (maybe our genes are messed up), I say ^£$" it, go with the feelings you have, don't worry about it, if needs be get a bit of space so you can appreciate her again, leave it a couple of weeks or so at most if things don't change you need to talk, there is nothing worse than pretending to be happy for the other person's sake.
Most of all, good luck dude.
(Okay so I repeated my advice a few times (I think the third iteration is my favourite) and I've only had one relationship and I'm at the beginning of my second, but that is what I've learnt so far, feel free to ignore it).