I have a difficult time listening to my fiance. Please give me advice.?

We have been fighting A LOT lately. And it all seems to come back to the same thing... My inability to listen. I mean, when I'm in the moment I feel like I am listening and may even be looking right at her, but when she asks me to repeat sometimes i can't remember and this annoys her to no end. I don't know what my problem is and how I can fix it. It is mostly over little things but because it is so frequent it has really started to bother her enough to were she has threatened to call off the wedding. I am not a comedian and sometimes i make terrible jokes. Tonight she couldn't finish her yogurt so i said "want me to fly it to africa and give it to the starving natives so you don't feel like you are wasting it?" and she didn't think it was funny and rolled her eyes and I said "you're so serious". Now the problem is we had had a fight about that a month ago or so and she got upset i said she was so serious because to her it wasn't that she is serious it was just that i am not funny. And so she got super mad tonight because i forgot about that last fight we had. I remembered the fight, I just wasn't thinking about it at that moment unfortunately. I just don't know what I can do to be more present and think more. I know my problem i guess, but as I said before I just don't know how to fix it. To fix weak arms you do push-ups. To fix a flat tire you fill it with air. She is at the end of her rope and I just don't know what I can do to better myself for our relationship :( I am reading a book called the seven principles of marriage for relationship advice and i thought maybe it was just my memory so i got a sudoku book to sharpen my brain.. i know the sudoku seems silly but i just don't know what to do. Please help me.