Right now I am just struggling to get the image of her and him out of my head
When I've been in this sort of situation, I take a different approach. I sit calmly in a quiet room, clear my head of anything else and then imagine my ex making love with whoever the new person is. In my mind, I visualise the most painful things I can possibly think about. I observe my physical response to this. Increased pulse rate, body temperature goes up and I start to sweat. Often tears run down my face. But I keep working through this visualisation. I do this several times a day, until one day, I realise, my response is changing. I still feel very sad, but my pulse isn't as high and I don't have tears running down my face. I am slowly on the mend.
Trying not to think about it, is like me telling you not to think about pink elephants, Suddenly, that's all you can think about. The mind doesn't understand "don't", it only understands thoughts. So telling yourself not to think about her and this new guy won't work.
Give it a go and see how you feel. Most of all, when you are having a really bad day, remember 'Like life itself, so too shall this pass'. You will feel better in time. I lived on the emotional rollercoaster with relationships until I was in my early 20's and at the time, never thought I'd get over a number of significant people in my life. But here I am. Still here and still 'sort of' sane!
MG