Seriously?
Time for them to baby sit then forget to pick up the doll for a few days! If they want you to know what its like to be a parent maybe they would like to know what it was like to be a grand parent. There is one other explanation I can think of for doing this is to make you not want a child,to make you feel better flawed logic . I personally hate those dolls. They kinda freak me out, It would be smashed the first time it started crying. I don't have a problem with babies just realistic dolls. Evil little Chuky doll things.
First of all, I applaud you for having a full-time job at your age and with your issues. It is difficult enough for some of us just to get by each day, let alone have a full-time job or full-time school schedule.
I think your decision should come down to the following: do I want to adopt a child one day? Am I going to have a family? On one hand, it would be a good experience for you as far as learning stress management - i.e., learning how to hold back and redirect your anger when a two-year-old is screaming at the top of her lungs and all you want to do is kick her face in for auditory-raping your ears...that skill is a good skill to have for auties and apies. On the other hand, if it is too much for you to take one one time, then you may want to kindly and gently tell your parents "no!"
As far as my parents telling me what to do after I turned 18, I'd tell them where to stick it. No one controls my life except for me. Yes, I will help pull my weight. Yes, I will honor them and help them with what they need to have done, because I love them. Yes, I will do everything I can to make them happy. But no one CONTROLS my life but me. If they love me and are going to help me financially, then I gladly accept their help and will do my best with what they give me. But I would rather live in a homeless shelter than with parents who try to control me. Parents that try to control their kids should...well...I can't say what I think should happen to them, because it's kind of gruesome and horrific. But when parents try to control their adult-aged children, it's a form of slavery. It always is, no matter even if they think it is the best thing in the world for their kid. I'll admit, I've seen cases where it was good for the kid - cases I couldn't argue against. But sometimes it isn't the best thing.
My bottom line: if you think what they want you to do is or will help you in your future, then take advantage of their challenge, even if it's being forced upon you. If it's too much for you, then sit down with them and tell them that you cannot do it. Better yet, if you have a P-doc or a school counselor, get that expert to explain things to them.
BTW, I moved in with two of my friends (who were married to each other) and took care of their four kids and cancer-patient mother full-time for two years so that they could go to work. I have learned my share of coping techniques, albeit some of them include holes in the wall. (But those holes weren't in children's faces, where I originally wanted to put them!)