You have gotten some good advice. Let me just add a comment or two.
You are having trouble telling which stones are alive or dead. That is normal for a beginner. And let me assure you that even players much stronger than you sometimes have that kind of problem, too. Usually because they think that some stones are alive that can be killed or captured.
The good news is that it won't take too long before you will be able to tell which stones are alive or dead at the end of the game with great accuracy. For now, you should just play on until the status of every stone seems clear to you. And make sure that all the dame are filled, except in seki, OC.
Doing so may cost you some points by Japanese (territory) scoring, but learning to tell which stones are dead is more important. If that bothers you, you can play by Chinese (area) scoring.
Let me illustrate with your last diagram. I suppose that it is Black's turn, either because White just passed, or because Black has captured one White stone. There are still dame on the board.