This e-mail is for when it's just too hard to say those "I can't see you anymore" words face to face.
I know this is probably a terrible way to be doing this, but I can't think of another way. If I say this to your face I think we'll both just fall apart, so I'm saying it the only way I can. I don't think we should see each other any more. We've had some great times together, but I think we want different things. You're looking for <other person's goals>, and I really want someone who will <your goals>. Those goals just aren't compatible. I think if we fool ourselves and try to make this work, we'll just end up unhappy. I don't want that for either of us. If you want to talk face to face, that's fine.
I don't mean this to be the last word. But someone needed to say it first, and this is the best way, I think. You can think about what you want to say and reply, or just call me. If you decide you never want to speak to me again, I guess I'll have to live with that. But please understand that this is as tough for me as it is for you. I care for you, and I always will. Let's talk soon.