Stop right there. Bike AREN'T dangerous. Its the soft squishy bit between the handle bars and the seat that is the problem.
Don't think about the speed, put some head phones in your helmet, listen to some music you like and watch the world go past.
I hate being pillion, its not me at all, i much rather the control. It might be worth going on to
www.geton.co.uk and seeing it there is any free hour trials left. Just so you can realise you don't have to be doing 120 all the time. Trials bikes are different to sports bikes.
Its not easy to skid, trust me, i fail at skidding regularly, I've done er 40,000 miles on the bike, and not lost it in a skid. For someone to pull out infront of you, the rider should be paying enough attention to avoid it, you can predict when they are going to do it and take steps so you don't hit them.
Who do you go on the back of the bike with, as its sounds like you don't trust their riding. is there someone else you can try pillioning with?
The best way to be a good pillion is to relax, remember the rider is in control, and if you are scared, ask them to stop doing what scared you.