They’ve changed but I can’t forgive them for the way they have treated me. Is this common?

Occasionally people can change but it can be very hard to forgive someone that you loved and trusted who has abused that trust. Just because someone has stopped being abusive, doesn't mean you have to stay with them. It's OK to realise that their behaviour has done too much damage to the relationship and that you can't move past it enough to be able to build a future together.