Where do I start? Only one thing? No way, there’s a list a mile long: 

I wish I was fitter, and stronger, and just liked sports and physical activities in general. 

I wish I wasn’t so grumpy and was happier more of the time.

I wish I could focus and concentrate on things.

I wish I had a better memory so I could learn more easily.

I wish I wasn’t so unsure of myself and what I know. 

But you know what? I don’t think I should choose. I’m gonna let my wife pick the thing she wants to change about me most, because she has to live with me. I think I know which one she would select!