What should I do with the personal device that my female colleague at work gifted to me after I severed ties with her because the relationship is forbidden, especially since I am using it?

You have done well by repenting from the forbidden relationship. You must remain steadfast, close the doors to temptation, and beware of the footsteps of Satan. The gift is permissible if it was a pure gift without a forbidden cause. However, if it was for the purpose of the forbidden relationship, then it is forbidden, and you must spend its value on the interests of Muslims and in charitable endeavors.