To start, discuss is a transitive verb, so it has to be used with an object. This makes both of your proposed sentences sound incorrect, to my ear. (They both make me want to ask, "not interested in discussing what?"). This is easy to fix: you can just say
I'm not interested in discussing this.
or
I'm not interested in discussing this topic.
As for 'interested in' vs 'interested to', I would advise you to say 'interested in' almost all the time. It is true that there are constructions in which 'interested to' is correct, but these are quite formal, specific, and uncommon. In this particular case, 'interested in' is correct, and 'interested to' is incorrect.
I should also note that, in casual conversation, the corrected versions above might seem a bit blunt. That's not necessarily a bad thing -- sometimes being direct is good. But, if you want something softer, here are some other options:
Let's talk about something else
I don't want to talk about [it/this]
It's a personal matter