Hi lemonandlime,
Welcome and hello

(Sorry I didn't see this until now). I'm glad you found the site and sorry for what brings you here. It could be that he has BPD, although even if he doesn't, and has what some here describe as "BPD traits," it can still mean a pretty difficult relationship. He had a breakdown and is taking antidepressants, and it sounds like he struggles to communicate well, and is self-absorbed, often a sign of someone who does not feel very validated in life.
Long distance relationships are tough even without mental illness involved, and it sounds like the relationship was getting strained even before the letter you sent. People with BPD can tend to have issues with object constancy, so just your absence (even if you lived in the same country) could be a challenge.
Is there a way to know if he is dating someone else, other than him telling you? Are you ok with it if he is seeing someone as a way to cope with the emptiness he may feel (especially if he is BPD)? It takes a lot of strength to not be emotionally injured in a BPD relationship, which means recognizing why he behaves the way he does and not taking it personally. That's not easy.
If you do reach out to him, and he's BPD, he may not respond right away, especially if he is with someone else. Is that ok for you?
And if you decide to keep lines of communication open, it may be best to keep the conversation light and not keep apologizing or feeling guilty about the letter. It's done, you apologized. Now is the time to return to the kind of relationship you two had when things were difficult. If he wants to dive into the problems in the relationship, you will know when he's ready

What was the nature of his breakdown?
LnL