I agree, difficult as it is to accept, you may not love him any more.
All long term relationships go through rough patches, but you haven't mentioned any real reason for this to happen. Something like a lot of stress, a new job, loads of things could make you feel distant, but just feeling like you don't love him as much, and being attracted to another boy, suggests that it's probably over.
A very similar thing happened to me with my ex-boyfriend after two years, I was more attached to my lifestyle and the idea of our relationship than I was to him.
It might not definitely be over, you can still make it work, but it sounds like you definitely need to reconsider whether you really want to be with your boyfriend anymore.