fellow adhd sufferer (unmedicated all my life for various reasons, the current one being the fact that i can't get an appointment with a specialist due to All This), and it absolutely ISN'T impossible for you to succeed, but it will be more difficult.
one of the biggest problems i had throughout school and then university was executive dysfunction - that is basically a difficulty with planning and "getting your act together", as it were. i don't know if you have the same problem but it's likely.
the first thing you need to do is tell your school/college that you're struggling to cope because of your adhd. mention specific things that you're struggling with, and ask them what help they can provide to help you do as well as you can. they should be willing to work with you.
the way i've found to stay on top of work is to-do lists. like, a lot of to-do lists, with VERY specific tasks on them. i basically treat my brain like a toddler that you can't tell "you need to get dressed" but "you need to get dressed. first, put on underwear, then a shirt, then trousers, and then your socks and shoes", because otherwise it's impossible to figure out where to start. (also, it feels great to be able to tick things off a to-do list.)
finally, make sure you are taking care of yourself. if you find that you hyperfocus on what you're doing when you get really into your work, set alarms for every hour, or hour and a half, or so to make sure that you get up and move and destress a little.
good luck!