How to stop thinking about it

Hi, I've been out of a long-term relationship for 3 weeks now and I can't stop thinking about him when I'm on my own. It's distracting and annoying and also sad. I'm okay when I'm with other people and have been doing all the things I would normally be doing, so I'm not struggling in that sense. I just think my brain isn't ready to let him go since we were together for 2 and a half years.
I know that it's probably normal to experience this but I'm not sure how long for and also how to just stop. Some days I think I'm over it and then the next I can't stop crying about it whenever I'm alone. I can't help but feel like this is a bit pathetic?

It is entirely normal, it is not at all pathetic, and three weeks is nothing after a long term relationship. You need to go easier on yourself. This is a process that you need to get through. It's not a quick one, but you will get there.

Its exactly the latter point, you're not used to it yet. Its not been enough time to get used to it. But you never will if you occupy your free time pining after them.
Time to incorporate your best distraction techniques WITHOUT them. You wont "just stop" its a grieving process.

Thanks I needed to hear that