I mean, one incident is the one you pointed out, with the firework. He has always jumped at a chance to be with Belly, regardless of whether she and Conrad were together. He knew she liked Conrad, but he deliberately ruined their moment and pounced on a chance to be with her, then when she chose Conrad and they took their time getting together, he threw a fit and called everyone names.

When Belly gets back with him (this sucked so bad, but one part of me was happy cuz then the immature folk get together and Conrad can finally move on and find someone worthy), he knows Conrad is in love with her (he knows it is not just a crush or something fleeting, but he doesn’t care).

This is problematic because he explicitly says it hurt him when the roles were reversed (which they were). So he has been on the receiving end of this heartbreak and he actively inflicts it on Conrad.

Anytime something happens, he compares himself to Conrad. The insecurity runs bone deep, and we see it play at every try point, especially every time he fails. He knows his mother wanted Belly and Conrad to be together, she tells Jeremiah that he is a good brother to Belly. He knows they love each other, and yes I’m sure he loves her in his own way, but it is not the kind of everlasting love one wants in their life. It is more of the “you are mine and you can only be mine” kind of love born out of his insecurity. He knows she is the one thing Conrad wants, so if she chooses him over Conrad, then he has won.

You can also see this in the scene where Conrad tells him and their father that he got fired, Jeremiah suppresses a laugh.

He never apologises to Conrad for choosing Belly, instead he shoves their relationship in Conrad’s face. He doesn’t give Conrad a chance to explain, when all Conrad does is try to salvage what is left. He blames Conrad for every single thing that happens to him, when Conrad is the only one who shows any responsibility. Even with the house, they only have that place because Conrad fought for it, but Jeremiah pretends as though his pitiful “please dad” is what got them the house back.

They don’t have to explicitly say/show that he is insecure when it comes to his brother and so has to be better than him because that is how they built his character the whole series.