well, having the same problem, I think the cure is to play on even if you have worse position, or in this case, where you have still better position, but worse than few moves ago.
I am a beginner rated about 1700. In my team I play 3 different leagues, of cca 20 games I have played, only two were against lower rated players. I am used to losing, despite my rating rises.
I realized that my problem is that I often give up after unsuccessful idea. I want to attack, my opponent defends well, I give up in my mind, stop concentrating and lose from even position. Almost every game I lost went like this. This sunday against FIDE 1900 I had a nice attack, won exchange, but underestimated his counterplay and got to the lost position (R+N+3P vs R+4P). I realized my problem and decided not to give up, but instead I took a little walk around, had some water and came back to the board. At the end I somehow saved the draw.
After one blown game last season a much more skilled opponent told me: after something "big" happens in the game, take a little break, go away, try to forget about the game, clear your mind. Then come back and look at the game with fresh mind and open eyes. He was absolutely right.
Like you. You won the queen and get absolutely winning position. If you could have take a break and return to that game 5 minutes later, I am sure you would win without problems. The same with the first game. From the possible rating gain I assume that your ELO coeficient is 30, so you are probably also a beginner. The point I gave is really classic problem among us.
Time and time again, I find myself screwing up winning positions. It's infuriating. Even if I have a large time advantage and a decisive material advantage, I often blunder and end up either drawing or, worse, losing. For example, look at the following two games:
Worse is the following game:
How do I cure my inconsistency? In that tournament, it made a difference of 40 rating points and it's just annoying to get winning positions and blunder them away.