I can't see how you screwed up. wiz used to have these problems at times, where it wasn't fair b/c this or that or the other happened. it took not very much for me to remove the item in question from WIZ, not from J or T - whichever he was complaining about getting more/better/whatever. this is called bean counting in our house after the Easter when J was almost 1. Each kid got an Easter basket then. It was the last time they got indiv baskets - we do a family basket, period. Why/ j's basket was the saem size as Wiz', but his had a lot more, esp a lot more candy. But Wiz just had to count how many of each color jelly bean they each got. And have a total tantrum b/c she got more of 2 different colors of jelly bean. forget he hates them. Forget he had a ton of types of candy she did not even have, and of many other things too. After being ranted at for a few min he was told that life isn't fair and i could remove his entire basket and keep everything for myself, incl the candy. he didn't stop and i did take everything.
As long as you feel 'guilty' for this, he is going to use this to flog you and get more 'special' whatever than his brother or anyone else had. He is using this, and your guilt, to hold you hostage. it is good he could verbalize and not have a tantrum, and that he invited you to play his game. that is really great. but he needs to learn that love is not measured in special things, and that life just isn't fair and he needs to get over that or he will be miserable for the rest of his life.
maybe this is strange of me, and maybe my focus is atypical and my reactions are too. but the 'you do more for x than for me' was aa huge peeve of mine. not a pet peeve, a herd of buffalo peeve. I often heard that Wiz got 'nothing good' and that we did nothing with/for him. Knowing J got a pack of gum could set it off. the book/toy/outing/etc... that he got did not count. I often would say 'poor baby. your mom never got you x or did y or took you to c or did d with your class' and each was something that i had done/boughtarranged for just him. i could go on for quite a while sometimes, dependng on the ridiculousness of his claim and how far he took the poor pity me. Twice he went really far and since he insisted that i hadn't given him ANYTHING that I mentioned, I asked if he wanted that to be honestly, truly the case? he was feeling his oats and said yes, what are you going to do about it? you are not allowed to do this and ths and this and this. so I went and dug all those things out and took them away from him. i gave them away or pitched them. they were not replaced, and he did not have a chance to earn them back,b/c getting them back meant the problem would happen again almost instantly. even if it was months later that he got it back, that still was the trigger to doing whatever the problem behavior was.
Stop buying into gfg's koi. it may be sibling rivalry, but he is not a toddler. Sweet pea is developmentally appropriate with her tantrums. Your gfg is not - does he want to be a 1yo again? Go back to only watching Barney and Blues Clues and that type of show? Back to no computers b/c he isn't old enough to handle them or play his video games/ or does he want the nice parts of being his age and to get over himself and this idea that it is your job to make things exactly equal for him?
btw, if he watns exactly equal, what does pc get that gfg would not want, as in to have a story for younger kids read to him, or to only be allowed to do what pc is allowed to do 0 which means losing any 'big kid' privileges that he gets for being older? His reaction to that offer might surprise you. I know Wiz was stunned to realize that if he wanted exactly equal, it would mean he got earlier bedtime, only to use certain computer software, no internet browsing even with filters or mom sitting by him, and his privileges went back to when he was j's age. She is 3.5 yrs younger than Wiz, and he did not want to go back in time to only her privileges. If what he really wants is exactly equal, give it to him. he only gets exactly the same food as pc, the same amounts of that food, same bedtime, no computer, no going out alone unless pc is allowed to, etc...
or he can accept that he and his bro get some different things and some the same things - and that bean counting just isn't going to be fed into in any way. You love them both, you see they get what they need and some of what they want, and you are tired of this koi. I would flat out tell him that, but not use the word koi.
You didn't screw up. he is manipulating to get 'more' b/c somehow he wins if he gets more. And for pc's no toppings tantrum, maybe next time he doesn't need the ice cream.
I'm sorry it was a difficult day.Was the computer game fun?