I'm so sorry. I know what a horrible feeling it is to know that you lost a betta because of something you did or didn't do. Last month I lost my first betta Ganymede; the filter intake cover must have come off and he got sucked in. It was so awful and I couldn't stop beating myself up for not checking ocassionally to make sure it was secure. I'm still not over it. :(
But the important thing is to know that accidents do happen. As horrible as the experiences are, we must learn from them.