At 22 your brain and skeleton aren't even completely matured, so 22 is far from old. In fact, a lot of people over 50 or 60 will tell you that they feel as young now as they did when they were much younger.
If you have painful memories and/or things that are making you feel unhappy now it's has nothing to do with "old". It has to do with "unhappy" (or "dealing with difficult memories").
Even people who are fairly happy in life, though, may tend to feel less than happy in their early twenties because they can feel like they got to the adulthood they'd been waiting so long to get to, only to discover how many more challenges there can be in order for a person to get from "just being an adult" to being "where he really wants to be in life". People often imagine how great it will be to get to adulthood before they're really there. Then when they get there it can be kind of a let-down in a lot of ways - at least until someone has some more time to build his life to be more what he wants it to be.
In this day and age, it may be even more difficult for young adults, just starting out, to get to where they feel as if they "have it all together" as far as their life and aims and happiness go.
If you have painful memories, there's at least the chance that you haven't had enough time yet for time to do its job of making those memories feel less acute. At only 22, there's also the chance you haven't had enough time to build up some new, nice, memories/elements in your life that can kind of "replenish" you "mental energy".
If you could do this, it may be helpful to find a counselor who can put some of what you're dealing with in a better perspective than someone as young as you is able to do. When particular big and painful things go on, or when too many bad things happen, when a person is young; the person is often too young to have gotten some practice with coping skills and perspective.
Best of luck to you. It sure seems to me that whatever has happened in your life has been a lot for someone as young as you to know how to process as well as someone older may be able to do. Time should help some, but in the meantime try to find things that may help you get your mind off what is painful, and try to focus on happier, or least neutral, things. (Give your mind a rest as much as you can.) We can't make time pass more quickly, so giving your mind a mental rest whenever you can has to sometimes be the next best thing.