Like Andrew said, often when I view my own work (with other people), I feel embarrassed for no real reason. Inevitably in every piece there's one part that I really love - an explosion that just rocks you in your seat, a fight scene with natural sounding punches - but for the most part, I'm just tired of hearing the same effect and especially the same dialog over and over. My wife says she can quote every movie I've mixed since we've been together since she usually has to hear the dialog looping for hours on end as well.
The other thing that tends to skew my experience is where the film is being shown. Everything sound good through my sound setup, and I know it's correct. Some theaters, expecially the smaller film festival setups in hotel ball rooms, etc, are not so good. For example, I did a nice mix for an action short. The director (without my knowledge), put in a piece of music that was 1) integrated into the mixed too loud, and 2) was very bass heavy. The room we screened in was completely skewed towards the low end resulting in not just the nuances of the mix being lost, but the dialog itself was nearly impossible to hear. I was mortified since I had been introducing myself as the post-production sound guy before the screening.
Regardless of how you feel, get some ears - both experienced and novice - on your work to make sure you're as on track as you feel.