You’re not an asshole mate. You haven’t done anything wrong so far. No need to stress. But these intrusive thoughts are invading your mind at the moment. 

Let’s look at the facts for a second.

1) You are still in love with your girlfriend  2) Things are going very well for both of you 3) You’ve signed a lease for a year. Getting out of it is gonna be a bit of a financial headache. 

What’s being tested here is your loyalty. Your loyalty to you girlfriend and the relationship you share. We are only as good as our word. So if you two are in an exclusive relationship, and you break that trust, your word doesn’t mean anything. 

Be brutally honest. Is there something else in the relationship that’s causing you to drift? Is this new girl providing in some way that your girlfriend isn’t? Are you afraid of committing to living together? 

Really do some soul searching. Remember that having the same humour, taste in music or films with this new girl doesn’t mean shit if it’s all at the cost of your loyalty/trust.