What is a toddler? A toddler is someone who, unlike a baby, has started to realise how small and dependent they are in the big wide world. As a consequence the main task of toddlerhood is to work out how far I can be in control of my environment, how far do I need to cede control, and is ceding control OK or will it be a disaster? This is what a child is working through in the toddler months.
Toddler parenting = all about balance. In some areas you need to be as unmoving as the Berlin Wall. But you also need to be flexible where appropriate. A child who isn't allowed any say or agency in what happens to it will be a very frustrated and stroppy child. Toddlers need to feel they have a measure of control and also to feel secure that, where they are not in control, the controlling person is pretty consistent so it is OK and safe for them to cede that control.
Be dead firm on the non-negotiables. My list of these would include: hitting/biting/mistreating toys etc. Chucking food around wilfully. Constantly trying to extend bedtime - stick to one or two stories or whatever it is. Being mean to other children esp smaller ones. Snatching. Grabbing. Constant whining for more and more sweets or biscuits - stick to your limit. Refusing to have hair or body washed. Refusing to get in car seat. Refusing to have nails clipped. With these last few if they won't cooperate just be firm and go ahead. You cannot allow them to be dirty or unsafe in a car.
Around food. You can't force a child to eat. Put things in front of them. Leave them to eat it or not. If they don't eat much they can always have some bread or a bowl of yoghurt. I strongly believe if you don't make too much of a fuss they naturally become more adventurous eaters as they mature. My DD has just turned 5 and is now eating all sorts of things she wouldn't have touched at 2. My son is 2.8 and still Mr No Veg or Fruit (though he loves smoothies and soup so thats what he has!). I think toddlers are programmed to be hyper conservative about food. No point trying to force the issue. They will eat more exciting stuff when they are older.
Doing it Self. If it's not a non-negotiable issue, then try to facilitate if you can. It's worth taking a bit of extra time over something or a small amount of inconvenience in order to give your child a feeling of control over her environment. Yes its a pain if your toddler wants to climb in the car herself or wants to spoon out her yoghurt herself, but if you have time, it is easier in the long run to let her do it. It will develop her sense of competence and confidence and decrease frustration. Of course you won't always have time to hang around while your child tries to put her own shoes on. But sometimes, try to make time.
What is a tantrum? They aren't being naughty/manipulative when they tantrum, they just can't emotionally self regulate. My son gets really cross when I tell him he can't have a third chocolate digestive. I don't think he is consciously thinking "Oh, I will really lose it so Mum can't hack it and will give in!" I think for him he is VERY cross and VERY upset about the lack of third biscuit because he REALLY REALLY wanted one, and he is 2.8 and very immature when it comes to ability to manage feelings, delay gratification and all the rest of it. So when your toddler tantrums, instead of getting drawn in and getting equally worked up yourself (easy to do, I know!) try instead to imagine acting as a container/processor for your child's strong feelings. Try to imagine lifting the unmanageable feelings out of your toddler and into yourself, because you can deal with them. Gradually your toddler will learn to self-regulate.