[pow_pp_-1_v]

Hey, I think, what you need to do is go meta. Think about the way you think. If your brain know when to use an interface or an abstract class, then the next time when you make a decision like that, spare a few moments to think about why. You might not get an answer but you will get hints. If you do this enough times, you will understand why you decided to do what you did. Thus you will will what's the difference between an interface and an abstract class.

I hope what I said makes sense. Try not to feel frustrated. Frustration never helps.