once I mustered up the courage to call befrienders and no one pick up the call. They are not 24/7 as they've claimed.

Anyway I skimmed thru your story fast, basically tough childhood yada yada yada, one thing lead to another, you did some fked up things leading to your gf left you.

It sounded more like you regret over your past actions and now feeling lonely without her. Usually if you could address the root of your sadness, you'll be able to find solutions as well. Maybe living your best life is a way to redeem/repent over the mistakes you have done in the past, if you can show that to your Ex that'll be the greatest apology you can give. Harm is already done and she can forgive but she can't forgets so you gotta move on.

For now, lets stick to finding new purpose in life such as getting promoted, new hobby and whatever is it that pique your interest the most before concluding you have depression. If all else fails, go consult clinical therapist.