What Causes Orgasmic Dysfunction In Men?

I have what I consider a serious issue. I m male and 52 years old. I was widowed in 2011. I had gastric by-pass in 2013 and have lost 165 pounds. I have started dating a woman who is 53 years old and extremely sexual. We have a fantastic sex life. I can satisfy her easily I m pleased to say. We explore many sexual outlets such as intercourse, oral and mutual masturbation. Now my problem is I can not achieve an orgasam no mater what with her. She says it will happen and its ok but I feel cheated. What can I do ?