Can you explain what you mean by 'discipline', especially in software engineering context? Is it doing test rigorously, code reviews etc? Or simple commited to deadlines and get things done?

Not op but: do skip useful parts of work you don't feel like doing. Do boring or annoying parts of work even when it is tempting. Do not skip test just because you don't feel like. Do clean up code after yourself even if it already works. Write documentation, ask questions even if you would rather code, etc. Finish tasks before moving to shiny new ones. When you know about bug, dont ignore it and at least put it into tracker. Send that mail to administration/management/customer, fill taxes, don't leave mess in common kitchen. If you promised you will do tasks in some order (e.g. prioritized them) follow what you said.

Don't come to work sleep deprived too often, eat properly, exercise. It may also mean taking time for socialization/family/help friends even if you would prefer to code right now.

Effectively, it means same thing as discipline in any other context.