Quick version.
F1 2010 is not a true simulator. GTR2 is.
Long version.
I'll explain it like this, I can't give you an exact reason but I can say it's "One of those things" type scenarios, I started racing a long time ago now, when I was a wee lad, on GTR2. I never raced in Public's at first I came straight into a Proper league, run very well, and I was just a backmarker, as most new lads would be.
Over time, skills, speed, ectera increased, had a stint in PT's, found myself consistantly top 5, then swapped to Race 07, suddenly I found myself as a backmarker again, drove Touring Cars, ended up top 5 again after a couple of months of practise, then started with the PrestoGP lads and I started driving F1 cars, and I was just a backmarker with occasional speed, think I had 2 or 3 top 5 performances in a season in a half, in short, I struggled.
Then at the end of last year we went back to GTR2 with the GTR2 race club, it was a GT1-GT2 enduro at Spa, one of my favourite tracks, 2 hour enduro, and I got a podium after my strongest performance of the year (and at the time I hadn't driven GTR2 for a couple of years).
That's my "One of those things" theory, some things your strong at, some things you struggle with. I've given up at F1's now and gone back to the thing I really enjoy - tintops.
Backmarker, running top-3, it doesnt matter where your running as long as your having fun. As long as your having fun and punching in clean laps, you'll improve. It'll take time. Wont happen straight away, but as experience builds, you'll improve.