I don't think there is anything wrong with pre-flight nerves or worrying about what could happen. For myself, it promotes that extra careful factor.
Before I started flying I had a fear of flying and heights even in a commercial airliner. I never thought I would ever do a PPL. It was only that I was bought a trial lesson as a present and realised how much I did actually like flying when I was in control.
Even now just over a year after getting my PPL, I still get the pre-flight nerves and still go through what if scenarios before I fly. For me, I feel this is a good thing and much better than jumping in an aircraft thinking that nothing will ever happen and have nothing but total confidence.
My own opinion is that generally the type of flying that I do is safe. I have had an engine failure and am still here to tell the tale. It wasn't anywhere near as bad as I expected it to be.
Everything we do in life is a risk and people die everyday from the most outrageous of things so there are 2 options. Either lock yourself away somewhere and just hope the roof doesn't cave in. Or go out and enjoy yourself while you can.