Assert oneself

to state with assurance, confidence, or force; state strongly or positively; affirm; aver:
He asserted his innocence of the crime.
to maintain or defend (claims, rights, etc.).
to state as having existence; affirm; postulate:
to assert a first cause as necessary.
assert oneself, to insist on one’s rights, declare one’s views forcefully, etc.:
The candidate finally asserted himself about property taxes.
Historical Examples

The desire to assert oneself is no less powerful, in the social interplay, than the impulse to submission.
Introduction to the Science of Sociology Robert E. Park

How is one to intimidate, persuade, resist, assert oneself against them?
Victory Joseph Conrad

One had to say something if only to assert oneself against that wearisome, passionless and crushing uproar.
Chance Joseph Conrad

It may be needful, on occasion, to assert oneself, or at least one’s authority; but it is difficult to do it without sin.
Expositor’s Bible: The Second Epistle to the Corinthians James Denney

At my age,” she straightened herself proudly, “one must assert oneself!
The Love Affairs of Pixie Mrs George de Horne Vaizey

I believe in such a place as London unless one has resolution to assert oneself people think one is a sort of shadow.
The Way of Ambition Robert Hichens

To exist is to assert oneself, to affirm oneself, to love oneself; he to whom life is a burthen rids himself of it.
The Essence of Christianity Ludwig Feuerbach

verb (transitive)
to insist upon (rights, claims, etc)
(may take a clause as object) to state to be true; declare categorically
to put (oneself) forward in an insistent manner
v.

c.1600, “declare,” from Latin assertus, past participle of asserere “claim, maintain, affirm” (see assertion). Related: Asserted; asserting. To assert oneself “stand up for one’s rights” is recorded from 1879.