My wife cannot reach an ?

We have been married for six years now, our relationship has been at the brink of divorce because my wife cannot reach an . At first I thought I was the one have an to soon, the fact of the matter is the longer we are making love, the longer she will take to have an . It's like following your tail, trying to reach it but never getting it! I feel really concerned about my wife and I don't know what to do. I tried slowing it down and focus on her, making her feel more pleasure and be more sensitive to her needs. We've been close but "NO S" from her!!! What am I to do?