It's easy to blame your "partner" but you have to look inward and accept a large portion of the blame.
You're lamenting the results of having a totally sub-centric relationship. Made her a princess to your Daddy/Mommy (your preference of role name) and are now upset because you have no control. Next time, because frankly, I think this is an unsalvagable cause, try a dom-centric arrangement. Where you get what you want and she loves you for demanding it.
Also, look inward and work on your self confidence. Saying chat is cheating is tantamount to saying reading erotica is cheating. That any fantasizing is cheating. As if engaging in such is robbing you of something. In fact, you say you had an excellent sex life. Maybe because your girl was whetting her appetite in chat and then bringing that appetite home to your bed. And when you puit a stop to it, you didn't provide an adequate appetite replacement.
Regardless, were you afraid you'd lose her? Is that why you tried to hold on so tight? Crushing the spirit like a bird grasped in your hands? Despite her obvious faults, you come off as needy and jealous.
Which raises the question, are you really a dominant personality. Learning all you can for her is laudible. But perhaps she grew disatisfied because going through the motions does not a dominant make. Look through the many similar threads here. Subs asking how to make their partners into doms. The tremendous number of failures because it's more than the activities of domination. It's also about the personality of dominants. Even those partners who tried often failed to be "dom/me enough" because for a submissive (versus a bottom) it's about being in the thrall of that personailty.
Sigh... perhaps you are really a service top. A submissive who knows how to give a good spanking at the direction of the bottom.
With apologies because I feel like I'm beating you up even if it's for your own good,
Oz