It seems to me that you have been as usual “swept off your feet” as this guy seemed all nice and mature and trust me girls do fall in love who is that old.
But after reading your query it seems that the guy was temporarily bored of his marriage and wanted something fun in his life so he started dating you and may be even fell in love with you for a short while, BUT you are too young for him and for some moments of fun he will not leave his well settled family and start over with you, this you have to accept no matter what.
Trust me when I say this a guy who really loves you will never blame you for anything that happens in the relationship for him it will be ajoint effort not the mistake of one person and also a person who loves you will not doubt you at all even if you stab in the back that is what love is.
This man seems to me as someone who is bored of his normal life and finds his outlet in smoking and drinking and may be you for a while.
And think about this would it have been practical for someone so young as you who has so many things to do explore and grow in her life to marry a guy who cant even be loyal to his wife and family.
If he really loved you he would have come by now
You might have heard this a lot BUT its time to move on, accept this fact consciously that he is not going to come back and remember you have done nothing wrong here even when you cheated on him with another guy, if he really loved you he would have never doubted you in the first place.
Dont blame yourself for whatever he got into like smoking drinking, YOU ARE NOT AT FAULT.
I would advice you please visit a shrink or a psychologist to help you move on dont think they are only for mad people therapists are for anyone who gets into something they find dfclt to get out of.
Please move on and do something to improve your life may be even join some spiritual yoga or something for peace of mind but move on and let him go out of your life and mind
best of luck