You're definitely not a disappointment. You're only in year 9 so definitely have plenty of time to improve before GCSEs. As scary as it may seem, tell your teachers you're struggling and ask for help. I'm sure they'd be happy to put on some extra revision sessions to help and maybe ask your friends if they can help you understand the work better. Try to watch some GCSEs YouTubers on the specific subjects you struggle in such as GCSE maths tutor, cognito, Mr everything English, Mr Bruff, first rate tutors, Mr Salle's teaches English, Mrs Whelan's English, free science lessons, science shorts, Matt green, primrose kitten, the history teacher ect and try lots of past exam questions. You could hand them in to ask your teacher to mark them if you can't find a mark scheme, you could borrow revision guides from the school library and you may even be able to find content creators on tiktok that could help to. Also trying different revision methods may help if you feel like that's the main problem. You can find plenty of info about different methods on YouTube too. I hope this helps. Goodluck! I believe in you!