Over 10 years ago he broke up with me. I've tried repeatly to break off all communication. I've tried reasoning with him, I've been a jerk about it and many other ways to get him to stop, but it usually last less than 2 weeks. Recently he has started dating someone (1st time since breakup). I told him I can't be that person who the new girlfriend always has to wonder about. I know him well. He didn't say anything but I'm sure he's freaking out internally because now its all important to see me ALL THE TIME! I can't take it. I have never stopped loving him & he is breaking my heart in pieces. Why is he doing this & how do I get him to just stop & go away?

He is only doing what you allow him to. Stand up for yourself and let him go or be his fool forever.

Ignore his calls, threaten to all the cops, and actually do it if you have to.

And make no more excuses about what you can't do.

You stop communication with him.
You don't answer his calls, erase any voice mails, delete texts and emails without reading. You don't call him, you don't text him for any reason at any time, even to tell him to leave you alone.

You allow him to communication by answering him at some point

10 years ago? You should be way gone. So should he.

It doesn't seem like you are trying THAT hard. Don't egg him on.
"I told him I can't be that person who the new girlfriend always has to wonder about"

Not sure what that means. His mistress? Or weirdo obsession?

If you need to call the cops, then do it. Sounds like a concern.

Don't take his calls, emails, texts, whatever. Don't ever respond.

Hope he gets the message. I would also notify your friends & family. Just so they have your back.

Better safe, then sorry.