Should I write to my ex?

It might be hard but maybe it is better to find a new circle of friends to hang out with. hanging in the old circle and with your ex possibly amongst the group, would just be potentially steering you into a situation in the past. Obviously, it looks like the relations btw u and the ex are not what many would see as "healthy" so it is really quite toxic to ociate with this person going forward and even though your common friends may not be the same, but for YOUR own sake and future welfare, it is better that you just create a new world for yourself with different people and then you will be able to move forth in life.