Sounds like OP and I are going through the same dilemma - I have been on a weight loss stretch myself since spring (well trying  )

I would recommend timing yourself on some trails; I feel like I'm slower than in the spring but in reality I am riding with more intensity (faster on sections / laps) and more often now (couldn't ride after work in winter) - so I'm getting burned out sooner.   

One thing that seems to help me mentally is that I have been incorporating a slower ride weekly (backing off intensity), and I am able to see proof that I can ride longer now than I could in the spring, even in the heat.

So when I get that gassed out feeling before I think I should I try to remind myself is that it's almost impossible to ride longer, faster, in hotter weather, more often, while losing weight, and uphill both ways