YES. I'm always undermining my progress. If ever I notice that something is particularly loose, I'll either think "it's stretched because it's ancient" or "it was never tight, even when I bought it". It's such a weird thing (although there's something to be said for being mildly dissatisfied, because complacency would lead to plateaus, self-sabotage and regression).

I've been working on strength this year. When I started a 30 day ab challenge in the summer, I was just about able to do the 10 second plank on day 1. By day 30, I was able to do the 120 second plank, with the increments increasing each day. I decided to keep up the effort, and managed a 2.5 minute plank, just to see how far I could go. I decided not to keep increasing the time because I felt my form was suffering, so now I do a 105 second hand-plank, 20 second rest, 105 second elbow plank pretty much every day.

I think those are the ways you can counteract your dismissive inner voice. I remember that first 10 second plank and how I couldn't possibly imagine doing 30 seconds, let alone 2 minutes or 2.5 minutes. I remember struggling through the 10-15 sit-ups and then doing 120 towards the end, in bursts of 20-30. You can't say "well, maybe I could have done all this on day 1, but I wasn't trying hard enough?" because that just doesn't make any sense. Now I'm setting new goals for myself and gradually moving towards them (all similarly simple, bodyweight-exercise types of things).

So try to focus on small, attainable achievements like that. I honestly believe that when you start taking your strength milestones seriously, you start taking your whole body seriously, and then you won't be able to deny or dismiss your progress.