I agree with Bizzy, you little toddle monsta is just trying to assert her indepence, pick your battles and give her some choices... if she doesnt want to eat breakfast so be it but if she asks for food 10mins later offer her back her cold toast until lunch time and then start the process again. She wont starve herself but dont let her fill up on fruit / biccies I guess!!!!
At 20mths she will understand you if you ask then tell then time out when you want her to something like get in the pushchair / change nappy. Diane Levy promotes 'emotional time out' so basically you get on with your day and things you need to do and dont give your child any of your 'goods and services' until she is ready to comply. So it doesnt have to be 'stand in a corner / locked in her room' type of Time Out if you are not comfortable with that, more like a gentle cold shoulder LOL Toddlers ultimately are very social and will comply to get you back on side - if you are consistent.
Also negotiation / bribery / distraction works quite well... if you sit in your seat nicely you can hold Mummy's lipstick / keys / money (50c coin works well for us lol) or we'll stop and look at the ducks / go to the park (tho not the best idea cos she has to get back in the seat)
Buy a set of reins and let her walk??
If you call DH in to help then IMO over time DD is going to learn that you are not the one in control and only Daddy has to be listened too.
Hope that doesnt sound patronising, just some ideas that work for us (DS is 2 at the end of the month). I think they feed off your energy too, if you are stressed and upset and frustrated then she will be reacting to that and escalating the situation.