The first thing I would if you were here, I would get you to go back into the state you were in as you considered loosing your life to cancer. And move it to the day when you found out you did not have it. That is a great gift you will always have as a source of motivation and gratitude. It is the same gift the ghost of Christmas future gave Mr. Scrooge.
I would then advise you to redefine your problems in solvable terms. Everything you mentioned has a solution, even though it may not seem that way in this moment.
Use the legal situation as a learning experience of what to never do again and nothing more. We all make mistakes, that is not the issue; it is what we do with knowledge of it that will determine everything.
As for the relationship issues, if you still love her and it is possible to mend it, do that. If not, adopt the belief that they were not the right 'one' for you, and that the woman of your dreams is out there waiting.
Rather or not you can stay in the band, if the members are truly your friends, they will remain your friends, and your ability to play the guitar will still exist. I personally would express my desires to the probation officer, who can certainly choose to allow it. If they say "no", I would petition the judge. If that did not work I would go to a psychiatrist and get them to petition the judge; as they will have more influence. A good doctor will recognize that playing is a great outlet for therapy and would probably be more than willing to help you.
Also, understand that if you truly 'ruined' your life, you would likely not be here to post this. If you have indeed 'ruined' your life, recognize it can only get better from here.
Again, redefine your problems!
