Why does my mother think I am immune to depression?

I kinda struggle with what I feel, like I'm pretty sure I have depression bc I've been hitting all the symptoms (e.g. mood, diet, sleep, motivation) this year and every online screening test is telling me to seek professional help. I'm a pretty academic student, my entire teenage years have been around studying and my grades are fine, and I have never, ever, thought about dropping out, no matter how hard it got (I have thought about suicide a few years back). But this year has just been so terrible, I can't keep up with things we're studying and it's not necessarily because it's so much more challenging, it's more like nothing is going in my head and it doesn't help that my sleep schedule is fucked. I've just had such a bad mood as an undertone the whole of this year, I always try to function like a normal person, laughing and hanging out with my friends during the day, but it gets really bad when I'm alone and I can't really help hurting myself. It still alarmed me when the thought of dropping out or taking a year off crossed my mind.

I guess the main reason I'm making this post is because my mother (a former counsellor) does not acknowledge or try to help me at all, and I've stopped trying to talk to her about help, I just act normal, bc she always pretends like she cares for one hour and then goes back to life per usual. Like when my parents found my suicide letter from when I was 14 they took me to eat morning tea and were pretty darn patronizing, saying stuff like "everyone goes through this" referring to things on my letter as my reason for my death, but I was honestly trying to find reasons for my sadness when I was actually just sad without reason. It's always very ironic when my mum tells me to fix things like I'm some kind of maths equation especially when she's so empathetic with her clients. It's even more ironic when she gets so worried when I'm physically sick.

I think she genuinely believes it is impossible for me to be mentally sick, or is doing everything to avoid it, although, I have been good at pretending, especially this year.

But bc of this, I often also question if I really have a mental illness, like I might just be too lazy to do school work, or maybe I'm being dramatic, like nothing drastic has happened in my life, am I genuinely a spoilt brat? I feel like if anything, I don't deserve to be depressed bc not enough has happened to me, and I'm probably just going through some phase. I think she might have a point sometimes, bc if she admits I have depression, I can just use that as an excuse to not do things and I think that's pretty bad, and I would have to get treatment, but I've never seen anyone with depression being successfully treated (maybe I haven't seen enough), so if I get diagnosed I'm basically stuck with it forever.

I'm also super stuck on this bc my mum and I used to have a very close relationship, I'd share my emotional frustrations with her, and she would share her raw experiences, I even came out to her. Anyways, she felt like the only person I could trust up until last year, where I was already going through the most competitive year of my life (where I really needed my family's support) so far and we had a very big argument and I never trusted her after that.