I am very sorry that your experience ended up being a negative one. Likely even traumatic.
It's easy for the emotional part of the human brain to blur the line of fiction.
Your pain is real, though, and that matters.

It really did get under my skin. It's an intense enough feeling that I *want* to call it trauma, but I can't in good conscience cheapen that word any further than it already has been. It's a video game at the end of it, pixels on a screen, and I'll be fine. I just have a lot of resentment about it especially when everyone's praising it.

I don't think it cheapens the word to call this trauma. Though I do understand your hesitation all the same.
Honestly, my personal experience is that some demons can only be tamed if you call them what they are. But if you feel this is different, then I will respect that.

I never really felt bullied, the monsters were all under strict orders to capture/kill any humans they came across and bring their soul to Asgore (to unlock the gate and free them, so technically good intentions), which is why of course you have to be the better person and work to get each to become non-combative. Many of the characters you meet even befriend the player on a pacifist run.
But then I did neutral trying for pacifist, then pacifist, and only went to the evil route just to see it through. Wow did it ever get dark.
I've been quite enjoying Deltarune, it has a much different take on the world and characters. I should really get back to it.

All of that would have been fine and engaging if not for that stupid flower always yanking me out of the narrative and judging me for my choices.

Well given that Flowey was the soulless husk of Asriel Dreemurr he was of course a complete sociopath and you're not supposed to take his judgment personally. If anything, it's actually amusing that on a genocide run, Chara (who was partially responsible for Flowey's cruelty in the first place) actually annihilates him.
Toby Fox basically made that summary at the end of the neutral runs as a way of helping the player to make better choices so they could go for the full pacifist run. I seem to recall other characters than Flowey chiming in too, though. I guess I was lucky that I did have a walkthrough's assistance so I knew if I even accidentally killed any enemy I had to reload a save immediately to maintain the pacifist run, so I didn't even know about that summary until I watched endgame videos on YouTube.

To be fair, I think the flower isn't meant to be a moral guide at all. I think you're "supposed" to resent him and reject what he has to say.
Even so, yeah, he can affect some people more deeply than others.

I'd been bulled and abused for over ten years at school and at home respectively. I know how that feels.

I had a very similar experience, just that I didn't finish it. I had very little experience in RPGs when I tried this, so all the twists and turns on the formula were lost on me, and even though I tried really hard to play pacifist in my first playthrough, it felt impossible, and then yeah, I just felt guilt tripped for the rest of my playthrough and eventually got tired of it and tried to restart. Of course, when I did that, it does that lil' twist, but it just struck me as very annoying, because it felt unavoidable from the start. So I just turned it off.

I do adore the music though, and having read the story I much prefer it now, but in a context of a video game, I really wasn't into it!

Not sure what you mean. But I guess it doesn't hit for everyone.

almost how went for me at first. didn't knew i could spare toriel by "doing nothing" and didn't knew i MUST put water on undyne, so i ruined my pacifist run