Mindfulness and being intentional go a long way in helping with this. You're probably in or around moments right now that you're going to look back and think the very same thing. I have to force myself to notice that.
You can build new things. Valuable things. Restore relationships or create something new. And these new things will be what you want to cherish. The only constant is that everything changes.
Do what you can now and savor it while you have it. Then build something again and cherish that as well.
You may not be able to fix the past, but you can amend it and build something worthwhile. Or, of course, create something brand new entirely.