3 year old constantly changing his mind

[Thighsaplenty]

DS is 3.5 and is going through a terrible phase. Lots of "challenging" behaviours but the worst are constant whining and changing his mind.
He has always been given simple choices (apple or banana/blue shoes or black shoes etc.) and this used to work really well. However, now he'll pick one and within 10 seconds, he'll want the other one. If I say no, it's the start of a whinging meltdown which can last all day.
For example, he chose a yogurt. Took two bites and then declared he wanted the other one. I obviously told him no and that he had to eat the one he picked or nothing and cue "but I wanted the other one" screamed and whined.
These struggles are constant throughout the day. Coupled with constant whining about wanting things, shouting instead of speaking, refusal to sit for meals (he just needs to play, apparently), I'm really struggling.
I've stayed consistent. He doesn't get his way when it comes to changing his mind. He gets told to use his inside voice and lots of praise when he does. He knows I can't "hear" whining and has to use his nice voice. Meal times are awful but if he doesn't eat/sit nice, he can't have pudding etc.
I just feel like I'm so negative all the time.
A lot of these behaviours are just for me. He won't do them to his dad or grandparents. It's really starting to make me feel like a terrible mum.
Any advice? I'm reaching the end of my tether and I'm starting to snap and shout and I really don't want that to become habit.

totally normal, I'm afraid.
it's a phase.
he's a 'threenager'

in the meantime you can make sure that he gets enough exercise (at least 1 hour a day of mad running about, preferably outside), good nutrition, and good sleep. all those can have great impact on behaviour.

I feel your pain - I had exactly this at 6.30am when I gave my 3.5yr old the choice of porridge or Weetabix: slight variation today in that he refused to choose and then had a lengthy strop when I chose for him (because obviously he wanted the one I hadn't chosen). And yes the whining and the climbing on things and the shouting - argh!!
Mine is also much worse for me, which does slightly reassure me that he will turn into a civilised member of society one day because he can obviously behave when he wants to....
I think first thing in the morning can be a bad time for choices when he is hungry and sometimes tired.
I would say I have also been fairly firm and consistent along the way, but it doesn't seem to have made any difference!
Sorry haven't got a magic solution but sounds exactly the same as in our household...

PS and yes I know what you mean about being negative: I feel like I'm constantly saying no... I do try to see any positives in his behaviour but it's hard sometimes!

Developmentally I think it's because he's learning that time goes in one direction. So that he can't go back through time to change his mind. It's a horrible lesson to learn because it means you have to make the 'right' decision every time.
My son is now 5.5 and out of that phase (it does get better!) but I tend to say 'next time' a lot.. so next time you can have the strawberry yoghurt but this time you've got raspberry. It's slightly softer than just no.

I remember my son crying himself to sleep one night about the strawberry/raspberry yoghurt decision. Kx

It's just the longest day ever when every conversation is a battle.
Guess it's just fine to dig in and hope that it doesn't last too long