My 1st day on the forum so I hate to chime in, but, I recently lost my beloved male Betta, Sparky. Here's his story so you can compare.
I used 3 meds, first Melafix, then week 2, I used Metroplex and lastly, Kanaplex. Kanaplex has always cured fin rot for me, but I didn't know exactly what was wrong with my fish. He continued to eat the whole time, but that was the only time he moved at all. After one month with no improvement, I decided it was time. He just stayed in one corner, at the top, and rarely moved. I could tell he felt really bad. I used the clove oil method and he passed peacefully. I would never suggest euthanasia. It's a hard thing to do and he's your baby. I considered my Betta's condition for a good while but finally did what I thought was best for him. Your fish looks pretty bad. Have you treated for fungus? I hope you will find someone who is familiar with his condition and that they can help you. Bless your heart, we try so hard for our finned friends, but there are circumstances beyond our control.