Hi!
There have been times in my past where it took me a long time to get over a break up, too, so I can sympathize with you.
Looking back on it though, the amount of time it took were not proportional to the length or the depth of the relationship.
You kind of hit on what it took me a long time to figure out:
“but I don’t know how to do that when my head is full of thoughts”
Boom, right there. You need to fill your head with other thoughts.
Certain kinds of people, myself included, fill our free time with rumination. This is very detrimental to the healing process.
How do you fix this? Fix it with whatever it is you love to think about that makes you happy! For me, I need to run, or lift weights, or rock-climb, or play the piano, or play trivia in a bar with my friends.
The healing effect increases when you mix in friends because you are no longer in your head, you are using your brain to do something and interact with people who didn’t break your heart.
Do you read? Join a book club.
Do you sing? maybe join a choir in a church.
You dance? Take a zumba class or something.
You will better yourself, meet new people, and get out of your head all at once.
It’s a lot easier to give advice than to follow it, but this is what I would have told my younger self: “Stop moping, go lace up your sneakers, breath deep, and smile. There are seven billion (minus one) reasons that you shouldn’t worry about the past anymore.”
Okay, you can think about it every now and then if you really want to, but you can’t worry about it.

Peace!