It is not abnormal that this would trigger depression. I would guess it would be abnormal if it didn't trigger some really sad feelings at least. As an introvert with one friend this must be particularly depressing and I am sorry you are going through this.

What about taking more time for yourself and your interests as a way to help and distract yourself? This will give you a break from your friend and possibly limit the impact on you. It is so difficult when those we love cause us hurt.