why does society expect so much from people like me?

I'll be frank. I have adhd and possible autism. I also have generalized anxiety disorder, depression, and other issues. I'm failing 3 of my highschool classes (I'm a sophomore) and my parents keep telling me so many things that won't help. They tell me if I just tried I could have all A's, they don't understand why I don't care about school. They don't understand why I'm so ready to throw my life down the drain. It makes me feel like to them I'm just some rebellious kid they can't get under control. I feel like they're giving up on me when they've done nothing to help with my mental issues. I've been begging for therapy since I was 12, and only my mom, (my dad is divorced and married to my step mom) who I've not asked once because of her financial situation, was able to get me therapy and even then it didn't last long cause my therapist moved to Florida and didn't tell me til our last session when I was only with her for like 7 months. I'm crying every night. I want to die so bad. I've felt like this for years. I've asked for help from everyone. I can't focus on school anymore, I don't see myself living past 18. The only help I get is that "Oh but it's a temporary solution" "think of all the people you've hurt" "your pets will miss you" which just makes me feel so selfish and so much worse. My only redeeming quality is wanting to stay alive to help my friends, but even then I fail at that and I can't do that when I'm constantly thinking of killing myself. I should have known that I would have amounted to nothing.