Welcome to NT!
I'm so sorry you are going thru this
First thing I feel compelled to say is NO you are not being selfish! This situation has affected you in a huge way. I truly understand what you mean about feeling like a part of you has died. Anytime we lose something or someone close to us, there is the grieving process to go thru. I'm sure you have a gammet of emotions hitting you. Please allow yourself this, and try not to be so hard on yourself.
Even tho you haven't 'lost' your boyfriend per se, but you have lost a part of him and a part of your relationship. This is quite a blow.

Is he receiving good medical attention?

There's also a forum for discussion on Depression. I've battled this for quite some time and it sure is good to have a place like this to come to where people really understand.

Please feel at home. Everybody is down to earth and the support is never-ending.

Also, your friend may have a good point regarding talking to a therapist. They probably will have some good suggestions and techniques that can help you as you are trying to salvage your relationship.

My heart truly goes out to you

Caring,
Rae