How wrong can I be?

I think I have made an awful mistake with this new job, I really do not think that I am up for it or am capable of doing it especially the taking bloods bit, I am not nearly strong enough to do that at all and mentally I do not think I have the strength for that sort of thing, what if I hurt somebody? or make a mistake? I don't know if I can cope with that. On a good note I did receive an application for client support worker (bank) at the Sussex Beacon so am pretty sure I will get that so all I need is for them to advertise for full time work and then I will be ok again. Just guess I have to stick with it and give it my best shot and then at least no one can complain and I can walk away with my head held high.