You need to have a long think about your future with this man.
By that I don;t mean thinking whistfully about how nice he is, and all the things you love about him.
I mean taking a cool and objective look at how your relationship is working - or not - especially in the light of the fact that you think you like someone else.
Obviously your interest in this other person has made you take stock of your current situation, and think about where it is going, and if it is the long -term relationship you always thought it was.
Your boyfriend says you have changed, and that phrase can mean a whole raft of things, from simple superficial changes, to life-altering changes, and you need to assess which of those have occured, and what the effect is on your future.
have a think about what you really want, how you see the future, and again, be realistic. It's no use looking for a happy ending if there is not a happy ending to find, so be sure you are looking at what you actually have,and will have, not just what you think you may like to have maybe, perhaps, could be ...
When you have your own mind straight, it's time to talk to your boyfriend and get his thoughts - no doubt he willl have been pondering on this as well.
If you make a decision to carry on, then forget the other guy - comitment is meaningless unless you simply avoid temptation.
If you don't feel able to make that comitment, then maybe it is time to look at your future, as a couple - or not.
Good luck - and keep us posted - there is masses of experience on here, so people will be happy to help you thrhough this.