How good am I ?

I wouldn't say you were really great, but hey, who is when they first start?
Quitting isn't ever going to be a good option, unless you don't enjoy the game anymore, just because you're not the best in the world doesn't mean you can't enjoy playing, and getting better can only come from that.

I remember when I was at your stage, not really long ago, and now I can pretty much 5* any song from any game on any difficulty on command. I could probably FC at least 3/4 of all the discs if I had the patience, but I prefer to just play custom songs now

The point is, if you're enjoying playing then keep playing. If not, then you can quit.
You're a pretty decent player anyways if you can pass the GH3 campaign, but moving from passes to 5*'s to FC's is a steep learning curve, so don't beat yourself up if you don't get it straight away.

Damn, feels like I just wrote an essay. >_>