There is some truth to the old cliché: "Time heals all wounds".
However there will always be a scar. As your life goes on other things consume more of your mindshare and you're sort of forced to focus on matters at hand.
There's nothing "magical" that happens it's all (gradual).
It's very easy to "romanticize" the past when we're unhappy with the present. However when we stop looking forward in life we start to die. Depression keeps us from moving forward. At some point after a certain amount of grieving it's important to (accept things) and start moving on.
"The woods are lovely, dark, and deep, But I have promises to keep, And miles to go before I sleep, And miles to go before I sleep."
- Robert Frost
Optimism comes when we look forward to the potential of tomorrow. Depression stays with us when we believe our best days are behind us. Your future lies ahead of you and not behind you.