All or Nothing

I've always heard eating too little is just as damaging as eating too much. But how little is too little? If I'm not really hungry what is a good choice? Has anyone ever heard of eating a piece of fruit at the end of the night to boost your metabolism? I tend to be a bit extreme either way- all or nothing and I'm sure it's slowing me up. The problem is once I feel like I've over eaten I go for broke with it!
I used to be able to eat anything I wanted on the weekends and behave during the week. It's been so hard to break that cycle, but I'm sure that's why I'm "stuck"
Any thoughts would be appreciated