Yes you are over thinking it, you're channeling your stress in the wrong direction. Its completely understandable, my Mother was attending college when myself, my brother and my sister where in need of a babysitter. She had to stop attending to watch us. She only blamed herself, because she realized she chose to have children without thinking about the responsibilities that follow. All three of us are adults now, and she has all the free time to do so.

Unless your in-laws were the reason you have children, I see no reason to hold a grudge against them. Mother In-law gave birth to the man you love today, raised him, and taught him the basics of life. That's the mothers job, and she did her job already.

If the goal for mothers is to have children, raise the children for approximately 18 years, just for those children to go out and have children of their own, for that mother to have to spend more years raising children.

Your a mother trying to raise children, and also do schooling at the same time. That alone is a major struggle. Now imagine 20 years from now, your children will have children of their own. Your children want to attend college, so they expect you to do the babysitting, as if you haven't had enough struggles to get them to were they are now. Now your expected to babysit more.

Doesn't seem like a fair deal to me. At what point is a mother allowed to continue her own life? Or do mothers end their life the moment they gave birth?

The Motherhood Rulebook says: Mothers are to raise and care for their children until they are old enough to take on life for themselves. That's all that title is required to do. This situation that you are having now is one good reason why women say mother hood is not easy. Its sucks possibly having to throw away your goals in life because of a decision you made prior.