Honey, I know exactly how you feel.
I'm the oldest girl in a Catholic family with 6 kids. My mother counted on me to help with my younger siblings and I watched my youngest sister so much she started calling me mommy. If my mother left to run an errand and left my father in charge of us kids...he would immediately find me and put me in charge. It still makes me angry that he would do that. I was given waaaaay too much responsibility at too young of an age.
When my mother announced she was having a 7th baby, I was so mad! I felt like I was pratically raising their last baby and I certainly did not want to do it over again! It turns out she lost that 7th baby to a miscarriage and although my mother was devastated, I'm ashamed to admit I was relieved at the time (but I was still sympathetic to my mom.) As an adult with children, I've had 2 miscarriages and I know how devastaing they are and I'm not proud of my reaction, but I was just a teenager in a difficult position.
It gets better. Being so much older than the other kids, you get to go to college first and I made sure to pick a college I needed to board at. I had soooooo much fun at college (never got in trouble - never drank, smoked or tried drugs) but to go wherever you wanted without kids or housework was heaven! To set your hairbrush down in one spot and it would still be there the next day - priceless! It was the best time of my life. I met my husband, there, too.
As an adult, I have a lot of fond memories of growing up in a large family and A LOT of practical experience. When my friends started getting married and having kids, I was the one they came to for advice and when I started having my own babies, I wasn't scared, I knew exactly what to expect. Now I have 4 and I'm careful not to ask too much of my older kids. As much as it sucks right now, you'll be glad of the experience.
Hang in there