I agree with Sanity & Lynn. You are hurting and need someone to talk with. The sooner the better.
I can relate to some of your feelings. I was born 7 and 8 years after each sister. I've felt that my birth was a mistake even though I was never told that. I was born before abortion was legal. My mother was Pro-choice. So I assume that if given the choice I would not have been born. I'm normally able to turn off that thinking when it comes up so its not overwhelming or whelming to me. I can understand how your feelings could be overwhelming and effect your relationship with your mom. Please seek help soon so that you can get treatment and hopefully your relationship with your mom can be repaired.