I sometimes talk to people that are not there, like imaginary friends.
But the strange thing is, it's people that are characters from TV programs (e.g. Wilson from Home Improvement).
When ever I have a problem with anything of any kind. I go outside in the garden and talk quietly talk about the problems and I can hear the voice of the person that is not there.

I have been to a counselor and they said "Your whole life you have been in situations where you can't find an answer to the solution with the problem. You couldn't tell anyone what the problem is because you think that they will find the answer so quickly that it will make you feel like an idiot. So you kept it to your self for so long that it is becoming a problem. Wilson from Home Improvement gave advice to Tim Taylor from Home Improvement when ever he had problems. You think is Wilson a good advice giver. One day your subconscious created a person in your head that gave advice in your favourite TV program."

That counselor gave me a reason why I am hear that voice.
But it doesn't explain why I am speaking to him out loud. My partner doesn't know that I hear the voice or why because I am afraid that she will think I am crazy.

Can anyone give me advice on how to stop the voice and/or how to tell my partner?

Yes, go back to the counsellor and ask. He/she already has info in order to help you.

It is not fair to your partner if you are experiencing problems and don't share them with her.

But how do I tell my partner in a way she understands?

You explain it to the best of your ability - you cannot control if and how much she "understands."

Go back to the counsellor and ask for suggestions. You can't think that asking questions out loud and HEARING voices isn't going to concern your partner. In fact, I think YOU should be more concerned.