Thought I was bi, now I am not sure

I came out as bi-sexual when I was 17. I have dated both women and men, more men though than women. I have loved both and been hurt by both. I am currently dating a guy (been together for a year now) and I have zero interest in having sex with him. I think about having sex with a women, yet I still enjoy all kinds of porn (gay and straight.) I work in a very stressful career and have a lot of other stressful things going on it my life as well. I feel that my partner and I are not on the same level in life. He is 4 years younger than me, but I also have always acted much older than my age (I am 28). I don't want to string my partner on if its not going to work, but he is a good guy that is very good to me. But everything said, I am not happy but I don't understand why or how to fix it. Does anyone have some advice?

You say " I don't want to string my partner on if its not going to work, but he is a good guy that is very good to me" You are stringing him along, and using him because he's being good to you. You fix it by being honest with him and end the relationmship.