People change, like seasons, and there's nothing you can do about that.
You can only learn to accept it because you do need to understand that you're just human being, and you have limitations. You can't control everything and everyone; yet you can control your own mind.
I know it hurts when you have to accept changes. It's not the change that hurts you, it's the memories of how things used to be that hurt. A mature person would understand that what happened, happened. The past cannot be changed, memories' gonna stay. It's the present and future that you need to work on, so focus on figuring out the best ways to move on.
Let go and move on - it's easier said than done, I know. But you have to remember that you're not doing this alone. You still have family and friends who love and care about you. There're people standing by you. There're people who believe that you'll be able to go through the tough times and get out of the storms stronger.
Things change, people change; but life doesn't stop for anybody. Life goes on, and you must move on. There's no "what if" or "could have"; there's only "it is".
More great stuff to read:
Why You Should Follow Your Dreams Now, Not Later
How Keeping A Diary Makes My Life Better