TL;DR at bottom. Also I’m typing this on reddit mobile so sorry about formatting.

So for some background, I have been friends with this person ever since I was 12 years old, we are both now 20. I’ve also always been a little bit in love with them and they are aware of this, but it’s never been an issue within the friendship, as the feeling was mutual, however, I wasn’t out as queer to my family yet. I’m very emotionally attached to this person, they’ve been one of the closest friends I’ve ever had.
My friend grew up under an abusive and emotionally manipulative mother. They were hard to hang out with because they’re mother would always make them cancel last minute for absolutely no reason. My friend got a full ride to any college in our state for being super smart, but they dropped out due to mental health issues. After they were finally away from their mother, everything kinda broke in their head, all the abuse finally got to them. They were diagnosed with PTSD, Bipolar disorder, and depression.
That was three years ago. They’ve moved around quite a lot, and have been trying to figure out how to live. They’re getting better and I’m very proud of them.
Now here’s my explanation for my feelings of abandonment. 
I used to text them about everyday, with either little or no response. I branched out to messaging them on discord, tumblr and other social media. Same kind of response. Sometimes I wouldn’t hear from them for over three weeks at a time. And I felt bad for messaging them so much because they’re struggling with their mental health. But they’d always be like “I miss you so much! I’ll talk to you soon! We should talk more!” Etc. And I’m unbearably loyal so I always respond enthusiastically and then I’m left on read for weeks.
At this point I haven’t heard from them directly in months. I only send messages once every week or so now. I don’t get any response. I’ve been feeling more angry about it recently, as I’ve seen them almost regularly post on tumblr (with LENGTHY additions to posts) and being active in group chats that we BOTH are in, almost regularly sending messages. And yet I never hear directly from them. I feel like we’ve drifted so far apart and I’m struggling to keep the friendship and I feel it’s completely one sided. However I feel like I can’t say anything because I’m worried I’ll cause harm to their mental health or that I’ll just make them worse.
Am I being selfish? Would I be the asshole for confronting them about my feelings? My heart aches when I look at all the unanswered messages I’ve sent.

TL;DR: my friend has a lot of mental health issues, I message them, get no response for weeks. However I’ll see them posting frequently online and active in group chats we both are in. I don’t want to overstep and possibly trigger something related to their mental health, but I feel abandoned.