Man, you see this so often... people are quick offline or with a certain group, they come here and it's harder so they quit...
I have similar experiences and I never give up.
Karting, I was pretty quick at indoor curcuits. My local indoor I was close to fastest lap and regularly on the podium. Then I went to Club100, and got slaughtered. Did I quit? No, just kept plugging on racing the quicker guys until I was running in the next class, then the class after that. You never learn from similarly skilled racing drivers.
The same is true in sim racing. I'm pretty quick online with AC against the masses, and usually battling up front for podium positions or at least top 6 which I thought was pretty decent. Call this the indoor karting track. Jump onto an AC race here with better, faster and more consistent drivers, and I'm mid pack. If I'm lucky... This is my virtual equivalent of Club100.
RF2 would prove this out even more so, and I'm well and truly a back marker, judging by this morning, although there were several factors at play which I won't bore people with here.
But do I quit? Do I get frustrated? No... not at all actually. I'm having fun lapping, (which you should already be happy doing or why are you sim racing anyway?), but with friendly alien-esque racers in RD that you can latch on to and see where they're braking and turning in etc., they also lend a hand when you inevitably start looking towards setting up the car, which to be honest should be the last thing on your mind apart from fuel whilst still learning. I've been sim racing for over 10 years now and still feel like a relative new comer to this awesome hobby. Things like this take time and tons of practice to even become a mid field runner. Kind of like real life racing...