As a frontend dev who also started from scratch at a startup, I can confidently say 8 months is barely anything. It took me about a year to not be a floundering noob, and a few more months to become largely self-sufficient. Keep working on the foundations, you will get up to speed eventually — it's almost like a law of nature. Focus on the basics first (HTML/CSS/JS), and move on to your framework only after the foundations are in place. Write some code every day, build some stuff on the side (nothing fancy required, mini projects are fine), and quite importantly, spend time debugging and understanding the flows in their codebase (I'm assuming you must've received KTs for explaining what the application is and how it works, which will help you in this part).
For learning the foundations, pick ideally one, or not more than two resources to complement each other (I'd suggest any good book for JS — you're free to research and choose whichever you like), and follow that. Don't get into the rabbit hole of finding better resources, it only ends up wasting time in the long run.
Side note, fuck that senior. I too had a senior like that who grossly underestimated my ability and sowed some self doubt into me. But on the positive side it gave me the motivation to prove him wrong :P Thankfully, after a lot of effort, I'm in a much better place now. You'll get there too, don't worry :)