Re: betrayal
First off, it is wrong of you to use the justification that S.S. was "not DAT into me". He obviously was into you otherwise he wouldn't be hurting right now.
Did K.S. know you were in a relationship when you were having sex with him? In other words, was he aware you were cheating on your then committed boyfriend to have sex with him? If he was not aware, you should tell him the whole truth. Then you can decide how "tremendous" your love is and if he still wants to marry you.
If K.S. was aware, he is a scum bag and you should definitely marry him so that he ends up cheating on you. This is the only way you can truly learn your lesson then really apologize to S.S. with deep humility and sincerity years later. Right now, you are feeling sorry for him out of convenience- to make your guilt go away so you can be "happy". It is very selfish. If you really deeply felt remorse for your actions you would not still be having sex with K.S.
You should be single for a while to figure yourself out. If you have any respect for yourself or S.S., you would stop seeing K.S. and realize exactly the effects of your actions on another human being. It is just waaaay to easy to find some other guy to distract yourself from your actions. And the fact that it is with the guy you cheated on S.S. with is, frankly, despicable.
Dump that loser K.S. and apologize to S.S. from your heart, then move on. This is the only way real healing will take place. Otherwise you will be living a lie.