Reading everyone's comments has helped me deal with my own situation a lot! Thanks everybody and, most importantly, thank you Rabbittarius for posting this! I hope you got help from this too!

Sounds to me like a classic case of Codependence...only way you can control the situation is ...every time they start to get petty...shut them out...don't give them fuel for their problem.

I'm curious to have an example of being petty?

I have this friend for 20+ years. Anytime I have good news, she can find a way to ruin it.
Anytime I have bad news, she lacks concern.
But we have been friends for so long, it's a bad marriage. How do we separate after all that?

If pettiness is part of the equation, is it really friendship?

Sounds like a frenemy. When you're tired of the merry-go-round, you'll get off.

I very much understand this, I do the same thing with someone. I've tried to get away from her on several occasions but then after a month or so I'll get an "I miss you" text. I then go back for the same bs. Currently on a hiatus from her and I think this one is permanent but I've thought that before.

Figure out why you are willing to put up with it and change whatever it is in yourself that's okay with them being petty towards you. Put up boundaries that protects you from any harm they would cause you. Then you can remain friends with them without getting hurt. Keep in mind, though, that when you put up boundaries, the other person will often be the one to end the relationship.

It may sound silly, but start writing down the details and dates of the cycle. When you get fed up again, confront them with the evidence you gathered. If the response is something like, "I am such a jerk, I let XYZ in my life affect how I treated you, I am sorry," Forgive. If the response is, "What sort of weirdo keeps calendars like that? Feck you and the horse you rode in on" get new friends.

Sounds like you have run across a narcissist. Run.

A person who acts in that manner is seeking dominance and control. He or she does not have your best interests at heart. You would be much better off without that person in your life.

Change the subject, or shorten your time spent with them, above all stop looking for ways they are petty....otherwise you are doing the same thing

My sister is a control freak. She insists on controlling every aspect of a person's life (including her kids). When I disagree with her, she refuses to talk to me for months. When she decides to talk to me I found out that she does that because she felt enough time has passed that she can be certain that I'm "back in my right mind".

Last month she called my oldest son because she didn't agree with something I was doing (it did not affect her or any of my children and was not illegal). Apparently she thought that by telling him, he would convince me to do things her way. He called me concerned, and I told him I had it handled. Nothing further was said except he told me to call him if I needed him.

My sister had no right to call him. The issue had nothing to do with him, and I was handling it. Because of that (and other things she has done) I have absolutely no plans to talk to her.

Not long ago, I ended what should have been a good friendship because the non-person was not only petty, but started to take advantage of me being a decent guy.