The best years of your life lie ahead; always. Tell yourself this little lie until it becomes true. It is a cope, sure, but a cope is a protective psychological barrier. Without the cope, we cannot do anything of value, and thus the cope serves a net benefit. If you believe that the future will only bring good, then you will seek for the good in everything that happens to you. I understand this is guru hippy self-help shit I'm spouting, but it works. I'm not religious, nor am I even that spiritual. I just believe that things in the universe happen to benefit me, even if it doesn't seem so.
Everything that happens is meant to happen and you were meant to suffer (yes I know I sound like I come from a weird Christian cult when I say that, or it sounds like I'm someone spitting out fragments of Buddhist philosophy, but it's a thought you should meditate on). Now you're here, you wish you did something sooner, but you didn't, and you can't turn back time. You can start changing now, though. If you want to fix a mistake that you made 10 years ago, the best time to start is now. It's the only time to start. Some people take very little time to acknowledge their mistakes, some, like you and I, take a bit longer. Some never recognise their mistakes. We are lucky to know of our own foolishness, and once the feeling of having fucked up passes, you can start working on yourself. It's ok to feel bad for yourself, but don't indulge it, because feeling bad for yourself ultimately leads to sloth, and is in its own way a cope to justify not doing anything ever.
Your regret can be harnessed into motivation. The things you didn't do needed to go undone, if that makes sense. If you did the things you think you should've done, then you wouldn't be here realising that you didn't do them, and if you weren't here recognising that you should've done more, then you would never aspire to want to do more. It's like Jean Valjean in les mis if you've ever read it; had he never served so long in prison, he never would have had the life altering experiences that lead him on the path to good. Your future is all up to you, and only you can fix it. You're capable of becoming "successful", whatever you define successful to be. Just don't define your success by your looks/age, that's a moidcentric view of the world. To me, I consider myself successful if every day, I contribute something, I grow a little, and I feel content. I think I am largely more successful now than even 2 years ago.