Start slowly and don't try to do everything at once. When you're feeling up to it (don't set yourself up for failure), try something very part time like a volunteer position or an inexpensive class. I mention a class because having to show up regularly and have your work graded might be a good way to test your coping skills for when things don't go well, and even if things don't go perfectly, taking a class still shows that you're trying to learn and grow which always looks good on a resume.
Edited to add: please take volunteer positions seriously. You would have to do something pretty bad to actually get fired from a volunteer position, but that doesn't mean you can't hurt your reputation by doing a bad job or being unreliable. I'm the volunteer coordinator for a local gamejam and while I try to be forgiving of people who are very young and have never held a real job before, I've dealt with volunteers who I couldn't honestly give a good reference for. That said, all you need to do is give as much notice as you can if you can't make your shift, communicate whether you will be able to make the other shifts you're scheduled for (one of my biggest pet peeves is when volunteers tell me they can't make it to one shift, so I give them a different one and then they tell me they can't make it to that one either), and try to make yourself useful when you do show up.
If the part time volunteer position/class goes well, try asking for more hours or taking a more demanding course and slowly build up to fulltime hours. I recommend going slowly because dealing with depression alone, for example, would be a massive amount of work. Depression + ADHD + personality disorder + treatment + job would be a totally unreasonable amount of work to take on all at once.
Building up slowly also gives you a bit of a history to point to (and potentially some references) when you start looking for a fulltime job and need to prove you're trustworthy. Don't look at your employment history as a bad track record from a terrible employee no-one should hire, look at it as proof that you can take stock of a situation you don't like, figure out what the core problem is, and fix that problem. It's really useful in a lot of fields to have someone around who will recognise it and speak up when things aren't working.
Depending on what kind of work you do, networking (kind of a terrible term for going out and being friendly) can help a lot. It only takes one good job to break the cycle, which means you only need one friend of a friend to take a chance on you.