I'm going to ignore my fiancee
Why would you ignore your fiancee?
I'm not sure if I want to marry her in the end.
If you're having doubts, it's best to approach them directly and discuss them as a couple. It doesn't benefit anyone if you just ignore your partner; that's just rude.
Fuck I just want to fuck around and cheat. No one likes to deal with sentimental people.
Well then I don't think you should be with her at all and you should tell her that. It doesn't sound like your heart is in it.