I thought I had forgiven my parents, and I am willing to forgive them, but their behaviour keeps stirring things up in me emotionally so I can't really function very well because I think they are both, in their own ways, being manipulative. They are not going to change now and I don't think that they are honest with me because I think they are in denial about some things. They both have people in their lives who live close to them who are willing to look out for them.
Neither of them are Christian - both have been involved in the occult. I don't live near them anymore. They can be nice to be around for a while, but the toll on my wellbeing has been immense - I am taking strong medication to help keep me stable.
At what point is it wise to sever ties?
You sever ties when you must.
First, don't justify wrong behavior with other people's wrong behavior.
There is no "well they did X, and were part of the occult, so now I can do wrong".
So you need to dump the 'they are not Christian thing', or the 'they were part of the occult thing'.
That said, sometimes you do have to sever ties. That is a very difficult and hard point to determine.
Years ago, a Pastor was teaching about how he met his wife of 30 years now, and how his parents at the time completely rejected her. They wouldn't talk to her. Wouldn't even talk to her on the phone when they called to talk to him.
This was damaging his marriage, as anything like that would. No one wants to be treated that way on a routine basis.
So he had a little 'meeting' with his parents. Showed up without the kids, and intentionally sat his wife down in front of them, and said to them openly that this was his wife, and he was married to her for life, and that she was not a higher priority in his life, than even his parents.
"A man shall leave his mother and father, and cleave to his wife".... all that awesome Bible jazz stuff, right?
He then said to them, that if they continued this, and did not accept her, that he would cut them out of his life. No more holidays, or birthdays, no more grand kids or Christmas presents. He would cut them out completely.
Now luckily this snapped them out of their stupidity.
But he was ready to say good bye to his parents for good. A man protects his family, even from his own parents.
If you must disconnect from your parents, then that is what you must do. But you need to be clear about this. You need to be sure that you are not cutting them out simply because you are selfish, and don't want to handle difficult people.
If you find that this is what you must do, then.... so be it. Do what you have to.
But there is only one catch. You must make sure that you still honor your parents. You can deal with them in an honorable way, with respect.... even as you start to put distance and space between you.
You need to ask yourself before G-d, am I treating them with respect and honor while I protect myself from them? It's a hard question. It won't be easy. But you can do this.