I'm sure someone else has felt like this but what did you do?
I'm currently 1.5 years into my first postdoc and I really don't enjoy it 
I feel totally out of my depth and genuinely don't think I am clever enough for what the job entails. The project is also very ambitious and because of this several things have gone wrong!
The first part of the study overran by about 4 months and for all of this time I couldn't do any proper work. I don't want to be too identifying but due to the nature of the study I ended up basically working as an unskilled worker. I worked single day (including weekends) from January to Easter before I broke. When I complained I was given help in the form of a technician which was fab and really helped for the rest of the study.
Now I am in the process of going through all of the samples/data which were collected. It's a disaster, all of the negative controls are positive, we don't have enough controls anyway (have brought this up several times).
The other person on the project (postdoc at another institute) is very well suited to her position and is getting on really well. Though she also thinks the project is doomed! At meetings it always looks like I've done nothing while she had bags of results.
Anyway, I think I just wanted a moan and to know what others did in this situation. I'm going to stick it out but now I'm questioning whether I should do another postdoc after this or do something what (but what?!).
I loved my PhD and this project is the same subject but different (PhD more epidemiology and postdoc immunology) so thought I'd be able to learn lots (I am) but I just hate it.
I've probably rambled and not made much sense but I'll come back after work and see if anyone has replied.