Sorry that this is long and rambling, I left stuff out to make it shorter. I just discovered this site and that my problem is common and I had to "let it all out"!
I am currently 20 years old. For most of my life, I simply avoided shitting away from my house and never really thought of it as a problem. Whenever I slept out, I would hold it in until I got home. Family vacations were never a problem, because my parents and siblings for some reason have never made me uncomfortable. It just never really struck me that shitting every day was an absolute necessity. I would go on camping trips overnight where I would backpack for miles with a "full tank". I also played 3 varsity sports in high school, including cross country (which involved a lot of distance running) after school every day having not shit since the previous night. Sometimes I would feel uncomfortable, but like I said earlier - I never really thought twice about it.
Once I got to college, things got more complicated. For about the first 3 months of freshman year, I did not take a single shit in my dorm. Every day, often twice a day I would walk to the academic building across from my dorm and take a shit in the basement bathroom. The building was never locked and there was usually no activity in the basement. I would occasionally try to shit in my dorm, but it would always be a misfire.
By second semester, I finally was able to start taking shits in my dorm. But the climate had to be right. If I walked in and other people were in there, I would wash my hands and leave. The worst was when people were in there just to hang out and not take care of their own bodily functions. Occasionally I would try to shit with other people in the room, but it only worked about 10% of the time.
My real issue is that now that I am in college, I spend long weekends visiting friends during breaks. I spent 3 days at my girlfriends house and had to hold it in the entire time. About halfway through, I woke up in the middle of the night to try to squeeze one out. The bathroom connected to the guest room was shared with her little brother. He was only 10 years old and it was around 2 AM when I was in there, but I kept imagining him walking in on me and seeing me shit and it made me clamp up. What's weird is that after a lifetime of playing high school and college sports, I've been completely naked in front of people in locker rooms 100000 times, yet I still felt pure fear that a 10 year old would see me shitting.
Lastly, a final strange aspect of my shy bladder is that it becomes much less of a problem when I am drunk. There have been several instances this year at school where I have been very drunk at a bar or a house party when the urge to shit strikes. If I were sober, I would never be able to go at a bar or crowded house. Yet, when I am drunk, I am able to pull it off about 50% of the time in difficult circumstances. I wish there was a medicine that would always make me be able to shit they way I can when I'm drunk without having to actually be drunk all the time....