The happiest time for me is not when I get a highobtain a senior position or when I make a lot of money.

Obtain sounds better :)
Senior usually refers to jobs / careers, rather than high. E.g. 'I obtained a senior position working with these people for many years'.

ThaIt is when I finally know clearly aboutunderstand myself, what is suitable for me, what I should do and love to do, and that I can do it with dedicated, peacefullyion, without being disturbed.

I had to make a few corrections regarding it vs that, but generally we don't use that to start a sentence how you had started it. Peaceful refers more to a state of being rather than a state of completing something (I think that's what you were aiming to convey?). Finally, to do something dedicated is technically correct, but stylistically you'll sound far more native saying 'with dedication'.