I know for myself or I know to myself

A somewhat formal phrase would be

I know it to be true of myself that ...

or somewhat less formally

I can say of myself that ...

“I know ‘to myself’" is scarcely English. ‘I know for myself’ works-it marks personal certainty, = 'as far as I am concerned.' That tiny preposition shift moves the sentence from clumsy to idiomatic.

If you're trying to describe a realization during self-reflection, neither of these phrases is actually the most natural choice. While "for myself" is grammatically better than "to myself," it usually sounds like you're talking about a personal opinion or a private conviction rather than a discovery about your behavior. Instead, try using "I realized" or "I admitted to myself." When you're reflecting on flaws like time management, you aren't just "knowing" a fact; you're acknowledging something you might have ignored before. Phrases like "I came to realize" or "I recognized" fit the mood of self-improvement much better. If you really want to keep the word "know," just drop the preposition entirely. Saying "I know that I have problems with self-discipline" is the cleanest way to put it. Adding "for myself" often just clutters the sentence without adding any new meaning. The best next step is to read your sentence out loud. If it feels like you're trying too hard to sound formal, simplify the phrasing. Plain language usually carries more emotional weight in a reflective piece.