Sex problem with partner.

Hi there. My question is about me and my life time partner, a woman if you wonder. We are permanent together now for seven years, and we know each other for eleven years. She is 27years and I`m 31years old. For the last three (3) years our sex life is not the way it was befor. It`s like she have no interrest in sex any more. Like for instance befor this problem aquer whe could have four -play for about 30 minutes the minimum. These days it goese on maybe for 5 minutes. I might be wrong, but there was a time when a friend of mine vissit her at our home while I was working. So I fount out about this, and was mad because she didn`t tel me about this. Since then then our sex life change. She told me nothing happend between them. But then why this change? I mean I tried everything. Some times it feels like I`m having sex with a doll. I asked her plenty of times if she is still happy with me, and if I`m the problem. She always answer no, it`s not me. One day I asked her if my pen*s is to small or to thin, she answer no it is hundred persent. Please can some one help me with advice.

I think I may be repeating myself,but in my experience you must first turn your partner's brain on first,also I can't say how much romance seems to be undervalued.
Try and give her a bath and get in with her and then give her a sensual massage;you should have her eating out of your hand.That's what im doing tonight!!!