I know exactly how you feel! I wish I could tell you how to fix it, once and for all, but I'm afraid your baby's behavior is...totally normal. I used to wonder which was worse, thinking my son was wacko, or KNOWING he was normal
I recommend that you read a few books on child development, just to familiarize yourself with what you can realistically expect from him. I like books by Don Dinkmeyer, but you can find a lot of info on the web, too. Just be cautious about advice you take. I know for sure that spanking never works the way we really want it to (or the way some people say it does), and neither does being bossy. As the adult in the relationship, it's up to you to be "in charge" energetically. Which can be tough when you don't feel well. I can tell you it is normal for kids this age to SEEM like they are trying to make you mad, but what is really happening is that he is individuating, or becoming aware that he is a separate person. But 18 month olds don't yet have the part of their brain that allows them to see situations from the perspective of another. They want what they want and really are not yet capable of understanding why someone else might be inconvenienced by it. Right now, your best ally is action. Words should accompany your actions, but it is unrealistic to expect an 18 m o to 'obey' (gah, i don't like that word). You will have to model the behavior you want to see A LOT before he will get it. For instance, if you want toys picked up, YOU must do it, hopefully with his cooperation, but if not, do it cheerfully. After a little while, he will begin to see that that is just what people are supposed to do. I believe we need to be more concerned with the principle of the situation, in other words, show him that you value the toys being picked up MORE than you value MAKING him do something. At some point (not yet) maybe around 3 or 4 you can start introducing the idea that if Mama has to take the time to pick up toys, for instance, she will run out of time for 'fun' stuff. Will he still pitch a fit? Probably, but if you are consistent and do not engage in power struggles, he will learn and be cooperative. Try to remember how very new stuff is to him.
He is also probably feeling your stress. Try to establish a regular routine-don't be anal, but remember that little kids need to be able to count on stuff like meals, snacks and bedtimes. It may well be unsettling for him to be away from you, but if you have to work, you have to work. Not everyone is able to be a stay at home mom, and not everyone WANTS to. If you do, and there's a way, great. If not, what you CAN do is mindfully find the very best care you can. Are you happy with your caregiver? Perhaps you can explore some alternative situations that would be less stressful?
I feel you, mama, it's hard to be mama to a toddler, and it's hard to BE a toddler, too. I hope this helps.