Discipline often breaks down into a battle of wills, but it has nothing to do with breaking their spirit, even as much as it seems the contrary. They're crying, you're crying, the house is in turmoil; "This can't possibly be good for them or anyone!" you think.
Discipline is not "breaking them." It's not mean or hateful or unloving. True discipline is about teaching. Plain and simple. And when you teach toddlers, you have to relate to them on their level, which is primitive. When they are pushing and pushing and pushing, you have to hold out longer than they feel the need to disrespect you. It's a learning process...you teach them what is okay and not...how you are allowed to treat people, and what is and is not acceptable.
Example: screaming for a bottle when you are trying to wean. It may seem like you are flat our torturing your child for no good reason (and what good mommy would ever do that?) but here's the truth about what's going on: the longer you let them scream for a bottle, the more you teach them that they can't do that (and that the world will definitely not end). BUT when they scream and hit you each night for 5 minutes until you give in, that is exponentially worse. True, you're giving it a try until it seems ridiculous to carry on, but in actuality, you are teaching them how to behave badly (versus letting them cry and teaching them that you can't be manipulated). When you give in, you teach them that it is okay to hit and scream - it works, and they get what they want.
Look, I know you are freaking exhausted and can't take another minute. But long term, you are creating a huge, never-ending headache. Parenting is about headaches, believe me. But those long term jobbers can be nipped by standing your ground now. It's not easy, and it really is every single day, because they are learning so much at this age. But over time it gets better and better. When you are consistent and firm but loving, your child will be happier, more secure, and definitely a joy to be around. This versus a child who grows up getting more and more demanding, tantrum prone, and guilt provoking...hmmm. Wonder which is better? Which child do you think is truly happier?