As long as you think you can make a change, it's better to do it "late" then not at all.. You might be an adult and regret a lot of your life, but there are still many years for you to look forward to..

Start at the small things you know get you down and work your way up; you shouldn't expect yourself to get a good job right away, but maybe try dealing with your ways of thinking as a start - think of a special word, and every time you find yourself comparing yourself to others, or telling yourself why it's hopeless, say that word and cut those thoughts off. It's ok if it doesn't work from the start and if you find yourself not being able to fight them off sometimes, but it's better to have less of those thoughts.
As you climb up the ladder, you should be able to reach better qualifications and get a more fulfilling job in the end. It probably won't be any time soon, but you can use that to comfort yourself and give yourself hope when you're down in the dumps.
As for friends - they're very good to have to help you find your way through your problems. Try looking around this very forum for them. Just like other small steps forward, try devoting a little time and effort each day to them, try to comfort yourself after setbacks, and if you know you'd do something destructive to your efforts, try to have a little space - a good friend will understand.

Remember - not everything might go as planned, but that's ok
My hope in you makes me feel you'll get where you best be eventually