These have all been questions in my head lately - so I posed the question to several people I know to read their thoughts on the matter. I've read blogs, skimmed articles, etc. This is what I came up with this morning.
Self-Discipline - is something we do. We train our minds, bodies, etc to do, or not do, certain activities or behaviors. Eating, exercising, getting up early. This is a lot of what New Year's Resolutions are about - and thus they fail.
Self-Control - this is a gift from the Holy Spirit. It is not something we can muster up within ourselves. None of the grace gifts can we work up within ourselves. So, a lot of times I need to not rely on self but rely on the Spirit to work within me the gift of self-control. I have this, but I let flesh get in the way. Because I have all of the Holy Spirit (not some of Him), but I let flesh get in the way.
Legalism is the same way I've thought of it in the past. Working (or not working) to improve my standing before God. My standing before God is all grace.
Just some thoughts to ponder!