McCloudington

I would definitely go with the original suggestion of resetting the phone. I also recommend you setting up a brand new Google account instead of loading your old one on. It might be a pain because you will probably lose some information, but restarting with a clean slate and fresh accounts virtually eliminates anyone’s ability to reconnect to your phone. If she had your Google account, she could at minimum keep track of the apps you use, business you do through email, and even locate your device with the Find my Device website. 

If she did download some kind of app or program that allowed more access to your phone like managing your messages or launching apps, the factory reset is going to remove those. To protect yourself in the future, set up a screen lock like a pin number or pattern. The fingerprint lock option is also a really good one if you have a phone that supports it. 

It sounds like you are in a really tough position, and it is only made worse by the fact that she has made you feel unsafe on your phone. I hope you get things ironed out on your phone, so you have at least a little peace of mind.