You're not a failure, it's just that lots of petshops don't care about animals at all, selling sick animals.
In this case probably to you.
That it is the 2nd in 2 months, makes me think you got it from the same place again?
It's a shame some care about money more then they care about a living thing.
I see it alot around here as well, when talking to them about something that's wrong, just gives you an indifferent or sometimes even angry answer.
Terrible how some of the stores deliver such a bad name to the pettrade.

Best thing you could do is never bring another penny to that place and try to find a respectable breeder or petstore.
You can tell by reading and asking around here all different things about the frog you like to buy,
this way you know the answers come from people knowing what they are talking about.

It may well be, that the info you got while buying the frog wasn't right, wich caused you to keep it in a wrong way.
Again, not you're fault, but that of the one making money on you.

Whenever you're setup is perfect for the frog you like,
go and buy one.
Ask all sorts of things, even if you know the answer yourself.
It helps you determine wether the seller knows how to properly care for the animal and gives you a better chance of aqcuiring a healthy frog when he does know at least the basic care.

Good thing you're on frogforum.
You'll end up a frog expert in no time

For this frog.... i think there is not to much you can do for it sadly enough.
These are signs the frog won't last long anymore.
You may try visiting a vet that knows a thing about amphibians,
but in most cases these are the final symptoms of something that needed treatment a lot sooner.

Hopefully it won't discourage you from keeping frogs.
They are wonderful pets.
Seeing that you feel this bad about yourself about all this,
i guess with some good guidance from other more experienced hobbyists around here providing you with the right information,
every frog will be lucky to end up with you in no-time