How do I secure my Apple ID if I believe it to be compromised?

Edit: I feel like a long story is confusing so I'll make it simple

Someone is using my apple ID, I know this cause there's history of app downloads that I don't recall doing

• I checked the device in used but there's no other device
• I have changed my password many times

Is there anyway to kick this person out so they can never use my apple id

(and even better delete whatever they did?)

[Re-Titled by Moderator]

Long story ver:

Hi so yesterday I was told from my college to download this chat app to be able to contact people from your class and stuff and this app was something I've never seen, heard, or downloaded. But when I tried to sign in using my Apple ID I got a notification saying its already used. I logged in using my Apple ID and saw all these chat being held with my name and I am super scared.

I got scared after this and went to icloud.com and checked and saw bunch of apps and websites my apple ID was being used for sth I've never done and I really want my Apple ID back...

I changed the password but I'm still worried cause it seems like they've been using them for 4 years and I usually change them every year.......

I think I know who it is that's using it and it's probably my ex... I dated him abt 4 yrs ago and he lost his apple ID and begged me to let him use mine. Soon as we broke up I removed his device and all the other account he was trying to use like google.... But I'm really worried he's still able to use my account and see what I'm doing please help