My protagonist finds out that his friends have completely forgotten him. I want to write an insulting scene for my protagonist and want the readers to connect with him fully. Need some tips on how to achieve this.
We can't come up with your story for you, but we can give you some tips how you might discover it for yourself:
Different people find different things insulting. So get to learn your protagonist and understand what is insulting for him.
People don't randomly do all kinds of things. So what is plausible to happen in the context of your story? What are the other characters likely to do?
Maybe start with researching how we humans relate to groups on an emotional level and how our relationships with those groups impact both our view of self and our place in the group. I'd think that a good understanding of age-appropriate group dynamics could be a bed rock for writing this scene.
Then, you have to decide how this group forgets about him. Really specifically. As in is this a case where the POV character vastly over estimated his membership in this cohort. So when they never reached out to him, reality collided with his delusions and his feelings were hurt
Of is this cohort of friends like any other cohort of primates, with members jockeying for position and status, trying to undermine others and elevate their own standing, without getting caught. Because getting caught negging a buddy can get you ostracized from the group. Not that I'm speaking from experience.
Once you've worked out in your mind the actions by all the characters in your story that led to this character being forgotten by their friends, you related them from the POV character perspective. How aware are they that this is going to happen before it happens? How important is membership in this group of friends to the character? How does he find out he's been forgotten? Does he try to rationalize the event away, telling himself it was just a mistake or a joke that everything is playing on him? Stuff like that. Losing valued relationships is no different than grieving the loss of someone who died. We go through the same steps, often multiple times, looping over and over through denial and anger. Not that I'm speaking from experience. We go through them in different orders. Losing a job, losing friends, impact us in very similar ways as death impacts us.
If you share the character's observations, expectations, fears, and revelations, then readers social programming will make the character relatable.