When I say “for myself” I mean it in a way such as “everything I do is for myself”. How can I phrase this in Latin?
The simple solution is to say mihi ipsi, which works independent of your gender. (Many expressions in Latin look a bit different for men and women.) In specific contexts there may be better choices of words, but for a general "for myself" or "for the benefit of myself" this will work, and it's also understandable as a motto if you wish to use it so.
Joonas' suggestion, mihi ipsi, is good but is somewhat emphatic: 'for me myself'.
I am partial to d_e's suggestion, mea causa; it is idiomatic without emphasis.
You might also consider simply pro me, a literal translation, on analogy of pro patria 'for the fatherland'. It sounds too easy, but it is attested.