First step is to forgive yourself. You might feel a lot of shame and blame yourself for being in this situation and that shame is counter productive. So first thing you do is wipe the slate clean emotionally and mentally. Without this step, you will keep getting off track. Accept the situation you're in mentally, but know that hard work can get you out of most situations.

Next step is set goals and do whatever it takes to meet those goals, no matter how small they are. The more goals you accomplish, the more you'll realize you can do it. Then set larger and larger goals, but make sure they're going in the right direction. Also understand that all you can do is put in the effort, and the outcome is out of your control. Sometimes the outcomes are great, sometimes the outcomes are not, but don't kick yourself over the outcomes as long as you put in the effort.

For me, it was in my early 30s. I had a shitty job, I went through a divorce, I was so fat I couldn't walk up stairs without losing my breath. I thought that being in my 30s there's no way I could recover by finding a girlfriend/wife because all of the good ones were taken. I spent about one month feeling sorry for myself, playing video games all day and then I decided that was it. I found a new job, I lost weight, I went to school and changed the trajectory of my life. I accepted that I was at fault for my part in the divorce, I said I would be a better person and forgave myself and moved on. I left the past in the past, and just looked forward to the future. Over the next ~20 years, I got better jobs, got married, had kids, etc.

One step at a time, and if you put in an honest effort, then learn to celebrate each win no matter how small, and move forward. You can do it!