The key clause which you forgot to add is; SOME OF THE TIME!
Nobody is disciplined 100% throughout their career/lives; it is psychologically impossible. If anybody claims so, then it is a sham and they are BS'ing.
It seems like you are seeking "external validation" too much. You are also just hitting the normal "learner's block" when you are not making any progress due to a) having mastered the simple stuff and are now faced with complicated parts b) having understood the interesting/core ideas but are now faced with the reqd. drudgery to be done to bring the project to conclusion.
The number one thing i have found that helps me maintain discipline when i need it, is to have self-confidence and NOT seek "external validation". You only focus on building/improving a little bit over yesterday's "you" towards a goal. Nobody knows nor is vested in you as yourself. By comparing only with your past self a lot of distractions fall away and you can focus on "your" goal. Focus only on the Primary core and disregard the Ancillaries until the core is done. Complicating unnecessarily (i.e. too many degrees of freedom) is a sure way to lose motivation. Pare away all secondary and unnecessary factors. Nurture "intrinsic motivation" (i want to do it) always and minimize need for "extrinsic motivation" (carrot or stick).
Here is a quote to keep in mind: "Drops of water, if they fall continuously, can bore through iron and stone".
You may also find the following two books helpful;
1) "Why we do what we do: Understanding Self-Motivation" by Edward Deci.
2) "Flow: The psychology of optimal Experience" by Mihaly Csikszentmihalyi.