Men and women find it difficult to let go of past relationships for complex reasons. It helps to have a little understanding of this. Sometimes we have unresolved grief from the past and trauma that has not been fully processed. It stays in our memory and activates when there are reminders or triggers. The end of a relationship means loss for most people and some times we need to do the grief work around previous losses as well as this one in order to move on.
Also, the loss is usually symbolic. For example it might be symbolic of loss of innocence or youth. It is really worth getting some assistance to do this work from a counselor so that it does not stop you from fully committing to a new relationship. Have some compassion for yourself also. Perhaps you have not fully acknowledged what the loss means.
Envy and anger are other emotions that can remain stuck and keep a person thinking about the past long after it is useful. These too need to be understood and acknowledged so that it is possible to begin again. If you find yourself holding on long after the relationship has ended, it is worth asking yourself what purpose does it serve to stay stuck in the past? Perhaps it prevents you from being hurt again in a new relationship. Perhaps there are underlying fears such as a fear of intimacy that get avoided by this focus on the past. Perhaps it is a fantasy or an idealizing of the past relationship that keeps you from moving on. Perhaps you imagine that it was better than it really was. Either way, it will help to acknowledge where you are now, the truth and reality of the whole situation and if it seems to big on your own then seek assistance from a therapist who can really help you to find your life direction in moving forwards.
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