My first two didn't last a week :( So, I went out and got a healthy new betta (shipment came in) and got everything brand new and devoted myself to keeping him alive. He did get ammonia poisoning on me, and I was able to treat and cure him. Now I have 5 boys, and I have no fear in owning Bettas. 3/5 of my boys came here sick, and they are fine now. You'll get there. I promise!
Don't overfeed, routine water changes, Prime water conditioner. Pretty much all you need to do.
There is a lot of information in so few words, I almost lost mine for the same reasons and thought I was doing everything right. Water changes, water changes, water changes!!!! Don't try to fix things with chemicals, just change the water. My little guy showed great improvement with just water changes alone. I too thought he was going to die but much to their pedigree they're fighters till the end. You can't always save them, but once you learn what not to do... early signs of a problem... and gain experience on how to treat them they'll thank you for it. I know my little guy couldn't be happier. I'm so lucky to have him, he has a strong will to live and be happy.
Water volume is your friend with any fish, so if you are using a smaller aquarium, consider getting a larger tank or doing more frequent water changes. This forum will help keep your betta alive when you're not sure what to do.
In the end, don't give up. They will fight just as hard as you do, if not harder.
Hang in there Kirby! You're not a murderer, it's okay. Fishkeeping is trial and error and sadly there's a lot of misinformation out there. You're doing the right thing by posting for help. Don't worry, with us helping you, you'll do better with your next betta.
Also, there are lots of other reasons why your fish may have died, such as the conditions they were kept in at the pet store. A lot of pet store bettas are sick but they don't show it.
You can so it, I know it is really hard and frustrating......... I feel your pain, my fish Skye has fin rot and dose not like water changes and he wile his is in the net he will jump out and land on the floor and there is nothing I can do about the fact that he jumps out, and now he is raggy but with a little perseverance and devotion you CAN keep your Betta's alive, just how I am trying with my fish, you are not a murderer, YOU CAN DO IT!