Hi, HD.
I am new to this site, but have already found a lot of good information. I found your thread today and had to respond!
I have very similar behaviors as you when it comes to wearing diapers when I am in turmoil or stressed out. I have done this since I was 4 or 5 years old as well as a way to calm down and relax.
I've always felt like I was the only one and never knew where to find more information on why I do this.
I've never heard the words "Transitional Object", but it makes complete sense! The only form of physical/emotional nurturing I had as a child involved my mom and diapers. I had a VERY stressful childhood. I had a very low self esteem, was illegally home schooled, was forced to lie to my friends about it, was a bed wetter until age 14 and my mother put diapers on my every night. At the time is was just another humiliating aspect of my life, but over time it became a comforting compulsion.
I had a blanket and sucked my thumb until that age as well. All of these things I associated with comfort. Like you, when I'm doing well and am relaxed I don't need them, but most times I only feel truly relaxed and at peace when I'm wearing them. Unlike you, it completely controls me and affects my relationship/intimacy with my wife.
Which is why I'm here. I need to find answers and hopefully a way to end the behavior.
Living Dead Guy made some helpful points as well...
Anyway, thanks for sharing and if you have any other info/insight please sure I found this very helpful.