You are not a bad fishy mama, you know why? You tried your best to save him, you helped him instead of just let him sit there! We all make mistakes and losing our friends does hurt, but it makes you just a little bit wiser for the future so you can give another fish a wonderful home.
Yes it is possible that he was sick from the beginning, and with the added ammonia, that didn't help. But don't worry so much of that, you've learned what to do so something like that won't happen again.
There could have been something else at work here, more than just internal parasites or swim bladder issues. So it wasn't your fault, fish die sometimes and we just don't know why.
Try to clean the tank out best you can so that it's nice and clean for your DTPK, does he have a name yet? Also, remember not to use soap
You can do it! I've got faith in you!