Turning off auto gears is in my opinion the easiest assist to lose. You will very quickly get used to manual gears, plus manual gears help with cornering... (just watch the fuel at places like Valencia tho as they tend to use more fuel).
Auto-brakes is a MUST to turn off, you will be so much quicker. You just need to practise to get used to it.
Racing line, while useful for new tracks etc, with enough practise at a given track you will learn to live without the racing line. Also I once read somewhere (probably here) that with the racing line turned on, you tend to chase the racing line even when you have come into a corner all wrong, & so the racing line when entering it wrong isnt neccessarily the best line round the corner...
Damage - turning this assist off, although it wont make you any faster, it does tend to make you more careful & a bit calmer, leading imo to better racing!
TCS & ABS. These will be the hardest I think to get used to, & props to peeps on controllers (rather than wheels & pedals) who turn this assist off. It just takes practise, eventually you will get used to it. I got lucky with TCS, I went to turn it off & found that it was already off. I'm still rubbish at braking.
Pit limiter & pit control is easy, you just need to hit the button before the limiter line, find out where the line is. & pit box control you just need to start braking when in line with the pit before yours.
Fuel sim on is rather rewarding I think, although you have to be a bit more careful at the start of the race, it is nicely satisfying seeing your lap times come down lap after lap.
Tyre sim, again no great problem, you are supposed to pit for tyres anyway, once again very easy to get used to.
Using flashbacks just interrupts your flow, even if you're flowing badly... try not to use them.
Basically it is all down to practise. The more practise you get the better you will be. When I started league racing here, I would often come last vs AI Amateur in career mode. I have only done 4 league races so far here, & have noticed the opposition tends to drive at legend speed. I have improved massively over the course of those 4 races, & I can now easily beat the AI on Amateur. I'm doing my practise racing in the Red Bull AI legend in single GP mode, as this gives me the closest performance to onlie racing, & similar times to aim for. As a result of doing it at legend, & putting in the practise for each race, I have improved. Still running last tho, so please do sign up for the league!
As for the races you see in the club section, the league races (certainly for PC) are the same ones as the real F1 weekend, & so as a result peeps want to practise the next circuit ready for the league race, which is why you keep seeing the same races as the real F1 posted for a week or 2 running up to the real F1.
You can always post your own races with whatever settings you want. I'm sure there are plenty of peeps about who would like to race but dont want to do so with all assists off.
Edit. Lol! wall of text. sorry