(((LittleForgetMeNot)))I can relate to this too. We want to cheer on our younger siblings cuz we see all this potential! And children are our future! And we love them and are happy to see them progress! But
us...is there really any point when we are so behind and bad, right? maybe? kinda? *SIGH* The good news is that you are still in highschool, so there is still time and potential in you too. I would try the school front office too. The least they could do is hand you some resources ("heres a pamplet & some numbers") to contact someone that could help. Or you may actually get redirected to someone willing to listen...You could also try contacting your "big sister". That relationship doesnt HAVE to just be "ima-take-u-to-the-movies-once-in-a-while-and-after-lets-eat-and-chat-for-a-bit" a couple of times a year. That relationship can include a lot of availiable resources on hand that she could give u to help alongside her care and just being there. While i see that you feel really alone, the reality is more like you actually aren't. IDK if this helps at all, could be somthing to consider......