Spending more time definitely helps, but I do believe the most important thing is to think.
When you mention that you analyse 1 GM annotated game, do you try making a move first, before seeing what the grandmaster has done? That would be really effective.
I understand that you have difficulty seizing the opportunity of mistakes your opponents make. If it’s a blunder, I presume that it is rather simple to convert it to a win.
What if it’s a strategic mistake? Even as a 2000 Elo player on chess.com I am still prone, occasionally to overlooking such mistakes in a game. Subtle things like isolated pawns and overloaded pieces are what can be taken advantage of. However,any often overlook this, giving the opponent the time to correct himself; it’s usually easier to see how you can win in your opponent’s shoes.
Hence, you should practice, playing maybe 20-30 minutes rapid games, and train yourself. Over time, you can get better and develop faster instincts.