Just RUN as soon as you get the opportunity and the emotional courage. The sooner you do it, the less pain you will go through, the more of your mental health will be left intact. This will save you years of grief and, possibly, therapy.
There is no reason for their behavior.
There is no conflict resolution.
There is no conversation you need to have.
There is no establishing boundaries.
There is no negotiation.
Do not look for logical explanations or closure.
That person is a 4 year old child, whether he/she's 20 or 40. Better to have a relationship with an actual adult.
But, of course, it is your right to decide to suffer for longer as well.
Sorry, I changed my diet. I don't eat crumbs anymore.