I know the feeling :( I lost my first betta, and I never really figured out what I did wrong. It's frustrating, and even with my new fish, I've seriously wanted to just quit. It's difficult knowing that little creatures completely depend on you for their well-being (in the sense that, they're out of their natural element for good.) I guess, for me, you just have to realize that you can't be perfect, and a LOT of things are completely out of your control. The only thing you can do is try. Even though it's an expense, and sometimes heart wrenching process, you have to work really hard at it.
I'm on the same page with Sowman, when I lost my first fish, I was devastated. I tried not thinking about it, but everyone knows how hard that is. I was so down for a few days, and then, I dragged myself out of the house, drove up to the local aquarium and decided to start FRESH. Buying a new fish really does help fill that void... and it helps with the disappointment.
I'm really really sorry for your loss, and trust me, everyone here knows exactly how you feel. I hope you don't blame yourself, because sometimes, there's just nothing you can do. I do recommend getting SOME sort of new replacement. Whether it be a new fish, or some other type of pet. Even sea monkeys make me happy. I hope you feel better <3