Unable to form sexual relations

[coldsunnyday]

Please don’t tell me to be happy single, as I’m very lonely and unhappy.

Why can’t I? It’s partly that I just don’t seem to fancy anybody, but I do have libido and desire sex.

We can't answer the "why", because we don't know you.

Have you tried psychotherapy? Or psychosexual therapy?

The one thing that struck me about your post is the title, which is very formally worded - as if sex is at a remove from you, as if you're somehow dissociated from it. If that's the case, then therapy would be able to help you find out why, and how you can reintegrate.