Is it possible to self diagnos nymphomania? I've read articles online. I had an episode across last summer where I cheated on my fiancé.

Not really. Nymphomania isn't just an insatiable desire for sex - that could be sex addiction if it has a long, increasingly severe history. Rather, in nymphomania there is also inability to reach orgasm. In your case, it sounds like you had a single affair lasting a few months? It sounds like it needs to be explored and understood better. I would see a therapist to figure out what the cheating was about.