Is it ever the bike?

[OakLeaf]

That's not exactly a slow bike, either, although it's true that fit could be off.

Have you ruled out all the usual suspects? You talked before about issues with nutrition (both on the bike and your recovery nutrition). That's a huge possibility. Also, it's a common mistake that people go too hard on their recovery days, or don't take enough recovery days. Are you following a training plan? There's nothing wrong with making it up as you go, but if that's what you're doing, are you doing a focused mix of speedwork, long rides and recovery? Sometimes a HRM can be a useful tool to enforce easy rides on your recovery days. What about form work (particularly downhill/high cadence intervals and/or one legged drills) once your fit is sorted?