Running Mommy,
I've not done tri's, only centuries so I have the advantage that I am not maxed out from all 3 sports.
I prefer to have less processed nutrition, bananas, maybe bars, apples/PB packed up in a ziploc (Skippy makes these little single packs!). But I find towards the last 30 miles I cannot eat, so gels are my only possibility eventhough they too make me gag. An alternative -- you can buy packs of Stinger honey for athletes -- honey is a little easier and goes well on a banana. Avoid too much fiber, not a pleasant result for long rides/races. The salt tabs or pringles probably serve you well but the Pringles would upset my stomach a bit. Consider adding some salt to your sports drink (I also like Cytomax) -- a little salty but workable. ClifShot also has those new "bloks" that are like gummy bears, I've not tried them but I love gummy bears (hey that might not be a bad idea, a bag of gummy bears in my back pocket...).
The mental piece took a long time to conquer as well, I had to learn how to really focus instead of letting the mind wander. And that's not what your instincts tell you to do. Try some yoga and such in your training...see if that helps you. I think it helped me learn to focus inward, ", after 40 or so, I take my next goal in 10-15 mi increments because you can do that amt in no time at all. Try and beat your next 15 mi increment then by shaving off some time. The games we play...;-)
Best of luck to you, I am cheering you on vicariously...I take off my hat/helmet to you for facing the IM! You go IronWOman!!