As already mentioned by Armiyana above, a year-old BP shouldn't be so thin after just missing a meal 3 weeks ago. We can suggest husbandry improvements on this forum, & share our extensive experience & knowledge, but with a snake that is so ill as yours, you should be concentrating on following your vet's instructions (or seeking a second opinion from another qualified exotic/herp. vet if you wish) but even with pictures, there isn't much we can do here to be of help. It sounds like a very serious infection that your snake has- keep in mind that a snake's circulation in their tail is rather poor, so medications however strong will have difficulty combatting this, I'm sorry to say. We're glad to have you join us here, but truly sorry that it's under such difficult circumstances.