After looking at one of the games you won without using any pieces other than your Queen, I am going to guess that is part of the problem.
You need to develop all your pieces and get your king to safety.
Have you done any of the lessons available recently?
I'm 18-1900 and still get checkmated frequently. You win some you lose some. It's part of the game.
Average rating is 600.
Usually when your king is being attacked and there is no way to stop or defend the attack. That's the typical reason. I'm gonna guess here but usually what was happening to me most of time when started out. Is I'd forget I'm playing someone who is trying to checkmate me, meaning I'd focus on what I was doing and not really paying attention to what opponent is trying to do.
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I used to be very good at this game and got to 400 rating but now im barely 300. I used to win most of my games but now I keep getting checkmated. WHATS WRONG WITH THIS GAME NOW? IT WASNT LIKE THIS BEFORE!