I am sorry this happened. It must be quite unsettling. If it were me, I'd trash my phone and buy a burner phone with cash. I'd start using that for everyday use and maybe only give the new number to people I trust.

If you want to test your theory that it has to do with her ex, maybe buy two burner phones. Use one exclusively to talk to her and the other for all of your other communications. If it gets hacked again you could then assume it has something to do with her.

You shouldn't have to do any of this, but that's a strategy that came to mind.

These Burner Phones Can Help You Evade Data Snoops | WIRED

If you are going to keep your phone and your number I'd cover up the camera(s) and remember it has a mic. Sometimes hacking can include watching and/or listening.
Here are some tips you can use to protect your phone in the future.

How To Protect Your Smartphone From Hackers And Intruders | Digital Trends

I know it feels invasive and infuriating, but try not to give this too much of your energy. If someone is playing these kinds of games, watching you destabilize is exactly what they want.