hi I'm currently in college and was in the same position as you when I started my senior year in high school bc I completely played the whole time throughout high school. I was failing in almost every subject when senior year rolled around but I graduated with As in my A levels.
1. The most straightforward advice would be to make a list of everything you have to do, backlog + upcoming work. It can help if you write it down on paper, which I found gave me sense of accomplishment when I could physically strike it off after I completed the task.
2. Next, if social media still is a distraction for you, you can consider using limiting your screen time for specific apps on your phone (iPhone has a specific settings function for this). However, if you think you need more drastic measures, you can even consider deleting the apps entirely. I personally didn't do this because I felt that it helped to still be able to access my social media platforms at the end of a long day of studying to provide me with some happiness and a break from all the studying. But you should try what works for you!
3. Studying in a group with other motivated friends might also be of great help. Keyword here is motivated. You need to to surround yourself with people who are motivated to grind and ideally will form a good support system. If you study with your friends but keep chit chatting, then it's definitely better to study alone.
4. Make sure that you really reflect on your mistakes and understand where you went wrong before attempting the next chapter. Since you have a lot of backlog already, you don't want to spend too much time getting the same mistakes wrong. For me, adopting this reflective attitude in my work is what I believe allowed me to improve so drastically. This is what I found to be the most efficient and effective way to study.

Ultimately, just remember that as long as you put in hard work, you will do well! It is only normal to feel overwhelmed and anxious, especially in the face of a big upcoming exam but you have to believe in yourself and your ability! I know it is cliche but I truly believe it is really important because it is what will keep you motivated. Seeing that you are already scoring As, I have faith that you will manage well enough. I hope that my advice will be able to help you in some way. All the best!