Wow! Lots of great responses, good insights, and good advice. Thanks everyone; this group is really helpful.
Reading the responses, the one that resonated the most was HomeyBa's - that my problem is mostly "between the ears". I remember reading a ride report earlier this year from another club member, a strong rider that battled 40 km/h head and cross winds for much of the ride, and ended up with a time of 11.5 hours. My first thought on reading that was, "I would never be able to do that." I know that kind of defeatist thinking is what ultimately gets me in trouble on these rides. When the going gets tough, I too easily entertain thoughts of quitting.
Eating/nutrition is my next biggest problem, and I'm still trying to figure out how to stay fueled up. After a point, I have trouble eating solid food, so I've been trying out more liquid calories with limited success. I can better tolerate food if I actually stop and rest for a bit before eating, but my "gotta keep going" mentality gets in the way.
As far as training goes, this year was bad for me. I broke my wrist back in September (collision with a car while riding my bike to work), and I didn't ride for three months after that (didn't do any exercise at all, in fact). I then started riding a stationary bike a couple times a week through the winter, and in mid-March, finally started doing some outdoor rides. In past years, most of my "training" has been my commute to work (10 miles each way), but between starting to work from home a couple days per week, and a bunch of other lame excuses, I haven't been doing much of that this year either. I also have a toddler, and a wife who has her own hobbies and needs time away from the kid to pursue them, and it becomes a balancing act trying to schedule in riding time. I'm not a 1000 km/month rider. I make the most of my commutes, and on days I work from home, I ride at least 20 km before work, so it's something.
As far as speed goes, I think my language in the original post might have been misleading. I don't think I ride particularly fast. In general, I ride at a perfectly comfortable speed, which for me is about 26-28 km/h on the flats with no wind. I don't use a heart rate monitor, but the only times I perceive a lot of extra effort is when going uphill or against a stiff wind. When it comes to wind, I resign myself to the fact that it's going to take longer, gear down, and try to be satisfied with a much slower speed. Hills around here are never very long, so I do tend to go hard on those. But I'm not riding at a racing pace.
And to clear something up...I cannot "comfortably" ride an 8 hour 200. The only reason I was able to do that on my last 300 DNF was because my strategy worked - I started at 5:00 am and beat the headwinds that picked up in the afternoon, and enjoyed some nice tailwinds for a good 75 km.
So, yes, I need to work on the mental aspect, which includes allowing myself to take the needed breaks to properly fuel my body. Thanks everyone, great discussion.