You are not being selfish at all. You are being true to who you are and that is the most important thing.
You are so young and have been with your boyfriend since you were eighteen and this is a time when you need to explore and decide what you want.
You have decided and now you must do it!
I know how you feel because I am in a similar situation and as hard as it is I know I must make the decision that is best for me.
If you were supposed to be with him at this time of your life you would know it. You wouldn’t be able to imagine your life without him in it.
Take your time and be true to yourself.