Hey and welcome Chrissy;
If it wasn't mental, then why would you feel so amazing during a dream? No sex for 5 yrs? Wow, I know that one. I really feel for you. Have you any idea of the cause/reasons? Have you tried discussing it? I know that sounds ignorant, but it does happen.
We fantasise with our minds because it makes us feel good. Our body responds most of the time just as it would if physical. The brain's in charge of this, it's not a choice.
If you see this fantasy as your issue, a visit with a psychologist or sex therapist might be better to begin with. That way you can be prepared for couples therapy or same sex connections.
Acting out fantasies can be quite daunting. People are real and act in their own interest, whereas dream characters are there to support the subconscious in giving you information. It's totally different. I'm not a professional, so my advice is only from experience and self insight. It's up to you which way you go ok.
Dreams can be very powerful when pleasure and emotions are involved. Talking about it can help to identify what's going on and why it came up at this particular time.
I hope I've helped, or at least given you something to chew on.
Talk here whenever you want. It's a safe and reputable place to vent or question life.