If you have been on Nemesis, you will be fine on Blivy and Rita.
They are all completely different from each other, but Nemesis is longer than both of them, and to be honest, I find Nemesis much more intense than Oblivion. Rita is more intense on the launch, but after that, it is fairly tame. Oblivion is fine. It is quite scary, but only because of the build-up. The actual ride lasts about 10 seconds, and so will be over in a flash if you don't like it. Rita is similar, the only intense bit being the launch. Just force yourself to sit in the seat, pull down the restraint, and don't think about anything until the train has left the station.
If you are scared of Oblivion, sit back-middle, and just keep your head back on Rita.
I'm sure you will enjoy yourself, I felt like this before I went to Alton Towers for the first time, I was terrified of Rita! I loved it, and as long as you get on the ride, you will have a great time ;D