Perhaps the 26 yr old came into your life just in time to show you a deficiency in your relationship with the fiancé that you did not realize before. Maybe you can work with fiancé to make things better. If not, you know the options.

As for 26 yr old, I personally just see him as a good time for you and not long term. Anything is possible but if you go with him I suggest keeping in mind that it may only be a short term fling. Your true match may be neither of these guys.