Definitely not just you, this always happens to me every single time. What I usually end up doing is hitting record and then just messing aroudn for a little bit so I don't feel like I'm actually recording, I'm just fiddling around with what I want to play and then when I get it right I happen to have it recorded.
For me the hardest part of the whole process is just starting. I'll sometime sit at my computer and not do anything for an hour. What ends up working best is if I just throw stuff down even if I know it's completely different from what I want and I'll end up trashing it all later. Similar thing to if you have to write a big paper and can't get that first sentence.