Forgive me if I'm wrong about this.
You've Already Decided to End It
But, I'm getting from your letter that you've really already have made up your mind that it's time to end it.
You've picked a good time -- graduation, moving on in your life.
You Have Changed
I don't think the question is whether he'll change, I think after five years you know each other really well. I think the issue is that you have changed. You are more clear about where you are going in your life and you are ready to leave the relative comfort of this old long-term relationship for more exciting adventures.
I Got Married to Save a Drifing Relationship
In my own experience, your feelings at this juncture are probably right on. As my college girlfriend and I drifted apart we made the mistake of getting married to "save" the relationship. What we should have done was break up.
Is that ironic? Yes, but I think you are having some of the same feelings by agreeing to live with him this Summer. My recommendation is to find a new place to live while you sort things out in your own mind.
I can't really advise you on how to break up. I've never been the dumper, always the dumpee. Therefore I suggest, since it has been so long, that you talk to people about the best way to "leave your lover." Or you could just listen to the Paul Simon song.
In any case, you're both in for rough times. But, I think that it is very common to spend your college years with someone, and then when you're done, to have real doubts. Explore those doubts. Then, whatever happens, you'll feel informed, clear, ready to face the future.