Re-train my brain from heartbreak?

When you do a model on a negative thought (or one you dont want to have anymore as it doesnt serve me), do you need to believe the turnaround thought/positive thought? or do i just have to keep ‘training my mind to only think on this positive one?
I am still in a massive (non-serving) thought loop after I broke up with someone a year ago. i am finding it really difficult to break the thought patterns around him and so i havent been able to move on.
I’d like to use the model to help if you could assist in more clarification on how i would do this? It hard to ‘believe’ in the new positive thought i try to work on, as habit (and my heart) still miss with and want to be him – so i assume i just have to re-train my brain to not go back to the old thought loop? and Just only concentrate on the positive/changed thought i put in the model?
Your guidance is appreciated!