Not 400 but I’ve been stuck at 600 for a year now. I play pretty consistently and watch videos but can’t improve.
After briefly looking at your most recent loss, my advice is the same as I shared earlier in the thread: Take you time to consider each move! It will make so much difference.
As an example from the game I referenced: After your opponent plays 9. Bg4, you play a move after just 5 seconds. Which cost you the game. Even though you had more than 18 minutes left.
Look at the position again until you find a better solution. In this specific example, you can easily find the move you needed to play. The only reason you missed it was because you made no effort to even look for it.
According to Stockfish there are multiple playable moves, but one is more obvious than others. If you can't it, you will have a hard time improving. Most likely, you can find it. You will be able to play the right move a lot more when you take the time to look for them.
whenever i reach 390s in chess.com i get defeated and sent back to 350s again and again
Nothing to worry about. Developing experience takes time and lost of games and it seems you're just getting started. Keep on playing and while you do that try to pay attention to what your opponent is doing,