I'm sorry about your Betta. If he was battling fin rot for months, the fin rot could have worked its way up to his body and made him more sick. If you're adding Betta Revive, you'd need to do complete water changes instead of partial water changes because with the partial water change, and still adding more Betta Revive, you'd be putting too much Betta Revive in the amount of water you have in the bowl. It's just one drop of Betta Revive to 16 ounces of water. If he's laying on his side, it's a sign of his swim bladder being effected, so it's progressed to much more than fin rot. When you give your fish a salt treatment, you want the water to be 80 degrees (a little warmer than usual) to facilitate the ingestion of the medicine. Did you check the ammonia level? Usually, the cause of fin rot is because the water needs to be changed more often so it's clean because otherwise, a lot of ammonia can build up in the water and be toxic. What happened with your little Betta?