How do you determine if you don't have interest in research or merely don't have the discipline? I finished my PhD in probability a few years ago and left to go work in the industry as software engineer in operating systems and databases.  I sometimes ponder whether I can go back to doing research, perhaps on a part-time basis, if I somehow transform my character to have more self-discipline.  When I was an undergraduate student, I really loved mathematics.  Even now, on my spare time, I frequently go over my old courses and, as Terrence Tao puts it, learn and re-learn old materials.  Each time it brings me great pleasure and newer insight.

What I have trouble with is actually spending time solving research problem.  With absolutely no intention of offending mathematicians, I find that most of the open problems in mathematics don't hold that much interest to me because I find them too narrow.  I really had a tough time with the proofs in my PhD thesis because they are hard and I couldn't sustain consistent work each day. I prefer discovering new connections and asking my own questions, such as pondering on the connection between complex analysis and probability or asking "what if I remove this assumption" and see if I can derive new theorems.
Even then, I usually can only do that for one day or two.  I simply cannot do it day after day like a professional researcher.  I would go back to my old habit of debugging and writing programs which I can do for 8 hours a day five days a week no problem.

Thank you for reading so far.  My overall question is, how do I determine if my lack of success in research mathematics is due to lack of interest in research or lack of self-discipline?  Can I just use something like the pomodoro technique and build up my tolerance for prolonged mathematics research?
First of all, you completed a Ph.D. in mathematics-- I would hardly call that a "lack of success."

Secondly, I'm a big advocate for getting paid to do what you're good at.  And from an economic point of view, that's the best thing you can do for society.  If you want to tinker with math problems in your spare time or build up self-discipline and concentration skills, by all means, go for it.  Such things will sharpen your skills in every aspect of life.  But humans are pretty straightforward creatures: we tend to pursue and get better at what we're interested in, and we flounder when put in career paths that just don't match our personalities.  You're not a lazy or unintelligent person for not wanting to do math research.  You already tried the path when getting your Ph.D.  If it didn't fulfill you then, it's definitely not going to if you try to force yourself back into it.