Is your diaphragm expected to improve? I know the nerve can be damaged or irritated during surgery and that it can take several months for it to repair itself. I would expect that this would impact your stamina quite a bit. Sometimes surgery can repair it. Is this something you've talked with your doc about?

Otherwise, your body has gone through a lot of trauma and you've been off the bike for 8 months so it's going to take awhile before you roll as easy as you did before. Kinda sucks I'm sure because it's not so long ago and you remember it being a lot easier. It'll get better. Don't worry about where you were, just focus on the progress you make from week to week.