I had a similar experience when I first started out and I got fired from my first two development jobs so maybe my experience can help you.
Getting good at grocking through existing codebases is an experience thing I feel. You need to understand the frameworks and libraries used, as well as understand the underlying business use cases. You can look through open source projects or build small example projects with the libraries in question to help you with understanding their code bases.
Understanding a code base is like understanding most other things. If you don't have an understanding of most of the background things, then you don't even know where you're struggling. It's like jumping into a book about quantum mechanics when you don't even know Algebra. You need to learn about the backend they use, the frontend stuff they use, and the business cases. I know that's a re-iteration of what I said above but sometimes having something explained from two ways helps.
As for the meetings and such, I think it is partially the above as well. I know when I first started out, not understanding the business, and the goals of the business, as well as what is in the code base already, nor what can be done in the code base, made the meetings hard. You don't know what they are talking about, why they are talking about it, and can't contribute due to lack of understanding. This makes it boring, like attending a lecture so high above your understanding that it's almost pointless to be there.
If this sounds like a likely scenario, then you just have to study and study and study. Also be honest with your co-workers and manager. Most of the time in my experience if you can identify the problem, figure out a path to fix it, and explain to your manager, they will be on board. Especially if you show that you are working to fix it. Always be introspective about the problems, and never be afraid to look dumb by asking questions. Also maybe confide in another coworker first about your problem if that feels more comfortable than your manager.
Lastly, I know that I had lots of these problems when I had an alcohol problem and drank WAY too much everyday. It made me sleepy in meetings, hard to think about problems, hard to understand new things, and just lots of other things. If you find yourself drinking a lot, perhaps consider doing a month of no drinking to see how that affects things. (I know you didn't mention drinking anywhere but I thought I'd say something just in case)
If you have questions or things just reply! Also maybe look at dev.to as well as they are a more junior community with a softer touch than here and may have lots of advice from people closer to your level or different perspectives that will help!
Good luck!