I'm falling for my Fiance's sons friend

I'm in my 20s and my fiance is in his 40s we've been together and happy for quite some time now we even have a child of our own together.. A year ago I got rid of some real heavy problems I was having and now that I'm doing better I'm seeing better. I keep thinking maybe I'm too young to be with someone so much older than me and keep asking will I be happy 10 years from now will I be happy walking next to my man with a head and face full of gray hairs... Then I would try to forget it well about a month ago I met a man around my age who has turned my whole world upside down he makes me feel so damn good emotionally and physically there are no strings or obligations which I love but now I see myself thinking more about him then I do my fiance and I feel like it might be a sign to move on but then it could be a phase and I need to stay with my fiance ..im so confused do about my fiance and