Perhaps if you can understand and make some sense of your current situation, you can overcome it or at least reach a level so that it doesn’t affects other areas of your life.
Its a common phenomenon seen among humans and animals alike. Probably your experience with this guy remained incomplete, may be you expected more out of it. So there are some residual desires still left inside you.
And may be he too is thinking of you too all the time, so you are catching up the thoughts and unable to let go.
The best and easiest way is to fulfill your wishes and dreams. Other way is to embrace abundance.
May be you know what went wrong with this relationship, so you can try to fix those areas. For many people its really fear and anger. So something can be done to let go of these emotions, empty it all and then you can relax and move on.
By embracing abundance I mean that abundance is truly our real nature. It is a state of consciousness. Since abundance implies everything is available in plenty, and desire/wishes implies absence of something.
Just like we cannot see oxygen/air with human eyes, but are aware of it. Same applies to abundance. Its there but we are not aware of it. Just remembrance of abundance will do.
Repeat this – I give thanks for abundance that flows to me and through me. My heart is full of Love and Gratitude.
Make use of breathing techniques, pranayama and courses offered by Art of Living. Meditation will also help.