This is actually one of my biggest regrets to this day. In fact, I kinda fall into the same category as a lot of the "friends" people are describing in this threat.
In high school, my best friend went to a different school than I did (i met her through one of my exes and we just sorta bonded and got really close). She started opening up to me and shared a lot of stories about her depression, self-harm, etc etc. I spent a lot of time just talking to her, and making sure she felt like she had someone.
The problem was.. I was young. I was stupid and I made some shitty choices. As her depression got worse, and suicidal tendencies became more of a reality as opposed to a 'what if', I admittedly began getting annoyed with all the negativity. I mean I had a boatload of problems of my own (all of which seem trivial as fuck now) but we seemed to spend so much time dealing with what she went through.
Fast forward a few months and I started to have a thing for this girl who was in my summer school physics class (she happened to go to the same high school as me). We started to hit it off and before you know it, we were dating. Unfortunately my new girlfriend didn't take too kindly to me having a female best friend whom I devoted a large amount of time to, and made sure that I stopped talking to her.
I'll never quite forget the phone call that took place where I pretty much told my best friend of 3+ years that I couldn't talk to her anymore because it would make things weird with my girlfriend. I'll never forget the tears and the anger that I could hear through the phone. I'll never forget her saying goodbye to me in what was essentially a verbal suicide note. I made sure to call her home phone and make her parents keep a watchful eye over her (her parents had no idea of the severity of her disorder), and luckily nothing happened.
Today, the two of us have made up. We obviously aren't as good friends as we used to be, but I've come a long way since my foolish high school days. I'm still dating that girl, and she has also since apologized and the two of them were actually good friends for a while. I feel bad that I'll never share the same bond with her that I used to, but I'm just happy she's still with us today.
TL;DR I am the guy most people are talking ITT. Don't be that guy.