@Cobweb121
From reading these comments seems everyone has angelic children. I often walk down the street and see other young children with their parents and they’re being good as gold, and I think why can’t mine be like that?
Their children are not angelic. The parents have trained them to behave in public. When they don't, there are consequences. Their parents are holding firm boundaries and dealing with difficult behaviour. The more a parent does this, the better the child behaves. It's cumulative. Just as it's cumulative that your son is ignoring you and running off. He gets away with it too often so he becomes more likely to do it. Saying "no don't do that" or shouting after him is not dealing with the behaviour.
My son is about to turn 3 and has his difficult moments as all toddlers do. But we deal with it firmly and fairly, and most importantly we are consistent in our response. It's tiring as we have to stay ten steps ahead of him and you're always fighting fires before they start, but that's parenting. It might look to others in a cafe that he's angelic, or when I'm walking down the street and he's holding my hand walking nicely and chatting. But that has taken, and still takes, a lot of effort to achieve. He wasn't born biddable. Far bloody from it. You're dismissing other people as having easy children and that your child is somehow more difficult. He's just your average toddler doing what toddlers do.
There is no shouting. We outline expected behaviour and there are consequences when that isn't met. If he becomes lost in a tantrum as toddlers sometimes do, we bodily remove him from the situation and when he's calm we outline the expectations and start again. This is a very rare event because we are fair and consistent and he knows what the boundaries are.
Your son should be on reins or in the trolley and given something to occupy him. Give him a little picture list of some of the things you're buying and involve him in it, even if that's just him sitting in the trolley holding a carrot for a while.
I can't recommend enough bratbuster parenting on Instagram. She coaches parents to become leaders to their children. It sounds like you're struggling to tap into that leadership mindset and so your son is running rings around you.