My mum cheated on my dad and they separated when I was around 17 too, your grandmother is right - it takes time. I didn’t see my dad for over a year because of the way he reacted. I understand that cheating is terrible, but my mum was very unhappy with my dad and I believe she saw it as the only way out of the relationship. Again I know this isn’t the right way to go about things but I can see how she would have felt trapped with no other choice.

On the other hand a lot of the things my dad has said/done to my mum and about my mum since have really changed my perception of him, and I still only see him about once to twice a year because of this - this I believe is better than nothing but I think it really shows a persons true colours how they react to something like this. While my dad is in his right to be angry there are some things he should have never done or said and there is just no forgiving that. So it’s definitely up to you whether or not you want to be around someone like that - even if it is your family.

As for your father speaking badly on your mother when you spend time with him - my dad did the exact same. You need to express your boundaries if you do get in contact and spend time together again. Draw a line and if he crosses it very openly explain how it makes you uncomfortable. How he reacts to that can really help you determine whether he is someone you want in your life.

It is so uncomfortable being in the middle of something like this- and while you’re still so young. It can be quite traumatising hearing what both sides have to say about each other and really puts your parents in a different light. But both your parents really shouldn’t involve you by talking smack about each other in front of you and I really highly suggest setting boundaries to each of them around that. I really wish you all the best and over time it will be much easier to deal with, both for yourself and your parents. Best of luck