If I had to guess, I'd say that things between you and your boyfriend are rocky right now. Clearly, you want to be there for him as he deals with his mental health issues, and this shows how much you care.
A person's mood and behavior can be affected by depression and the medications they take. Your boyfriend's emotional distance and coldness may not be indicative of how he feels about you but rather of the difficulties he is having managing his mental health. You should tell him how you feel and find out how he is doing emotionally. Make it clear that you care about him, but that you must establish limits to protect your own well-being.
Separation can give you both a chance to think things over and tend to your own emotional wellbeing. Be sure to discuss your hopes and fears for the future openly and honestly before you part ways.
In addition to supporting him in maintaining his current medication and therapy regimen, you may want to consider doing the same to help you manage the stresses of your long-distance relationship with someone who suffers from mental health issues.
Supporting your partner is important, but so is taking care of your own mental health. It's healthy to put yourself first in a relationship and establish healthy boundaries.