Here's what I got so far:

You can find one of me in a pentagon, but two in each of the vertices. After a point, my beginning adds up to my end which is identical to my middle. What wasn't used is the same before that point. What am I?


I'm fully aware that this might be completely retarded and not how riddles work but I'm taking a stab at it anyways. Might be too esoteric. If no one gets it i'll post the answer in an hour and a half.