Thanks for your quesion. I hope my answer brings you confidence that you can help yourself and a direction in which to begin.
I would like to tell you about a therapy called EMDR (Eye Movement Desensitisation and Re-Processing), which allows us to re-visit and let go of any beliefs that have been lodged in our subconscious mind from when we were younger. When we are asleep, we enter what is called REM (Rapid Eye Movement) sleep, where our eyes move quickly from side to side and we process away any stress or anxiety that we have experienced throughout the day. However, if an incident occurs when there is a high level of emotion, the memory can become lodged in our memory in a place where it isn't naturally processed away through REM. Sometimes these can be positive memories (eg. winning an award, meeting a new boyfriend, etc) and we can often recall those times "as if they were yesterday" being almost able to touch the memory, noticing details of the surroundings, sounds, textures, even smells. We can use memories of these positive times to increase confidence, etc. However, if the memory that is lodged is less positive, it is often pushed back down and continues to exist, even if we are not aware of it. Later, when a related incident occurs, the beliefs in the subconscious mind can be triggered and the emotions linked to that memory can resurface, stopping the behaviour from happening again. It is important to know that the subconscious mind does this in order to protect us - it is the reason why some people are afraid to fly, stroke dogs, etc. The subconscious mind believes that there is 'danger' and makes the person uncomfortable in order to stop them in their tracks. EMDR therapy allows the client to re-visit the memories, recognise the irrational beliefs that stop them from doing things they want to do, and to let them go.
In your case, there would need to be some adaptation of the therapy because you are unable to recollect the original memory, but the therapist will be able to manage this for you. You would be led through each step of the therapy gently and at your own pace. Once the beliefs have gone, the therapist will probably encourage you to visualise having sex in the future, which you will be able to do without the related negative emotions. You can then make this a reality with your partner.
If you look on the internet for an EMDR therapist in your area, you should be able to locate one.
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I wish you the very best with this issue.