Betrayal does not necessarily imply either malice or deliberation so I am unsure hex work is appropriate. However, binding the individual to prevent them from doing you further harm would be a good starting point. You dont indicate your own level of comfort with using magical practises so I will give you a basic idea of how to perform a simple binding. (There are of course many different ways to do it).Take a small piece of melted wax or plasticine and form a simple representation of whoever has hurt you. Try to personalise it in some way, hair is ideal if you have it, if not try inscribing the name of your person on the figurine. Then physically bind the figure with string, ribbon, anything that will tie. Sometimes I like to seep the string in salt water first. Visualise the power of the person being limited with regard to yourself. Perhaps imagine a shield between you, blocking their negativity toward you from leaving them and reaching you. Seal the bound figure in a jar or cotton pouch.
I would advise from a protective stance that you ensure basic general protections are in place. Do you have a witch bottle buried? Are your own shields in place and strengthened? Have you put any protective barriers in place on your property?
With regard to removing the person from your mind, I would suggest something as simple as writing a full and frank letter detailing all the problems you have with them, the thoughts that are occupying your mind and the power you still feel they have over you. Then instead of sending the letter to them as a physical letter hrough the post, burn the letter and send their negative energies back to them through the smoke of your burning.
Image http://www.sxc.hu/browse.phtml?f=view&id=1418837 (Stratsan)