자신 (jasin) has two main meanings > self/oneself, and confidence. Depending on the person used in the sentence, it could mean oneself, myself, yourself, himself, herself, themselves
자신 has a meaning very similar to 자기.
“Oneself” or “Self” (自身)
Refers to a person themselves, emphasizing individuality or self-reliance.
자신을 믿어야 한다. → “You must believe in yourself.”
“Confidence” (自信)
Describes a strong belief in one’s abilities or success.
그는 자신이 있다. → “He has confidence.”
If a person is mentioned twice in the same sentence, but not both are used as subject, then 자기 is used.
If however both times the person is mentioned as subject, then 사진 is used and 사진 is used as pronomen.
The standard translation is oneself, but depending on the person the translation could then change to myself, yourself, himself, herself, themselves
저는 자신을 안 믿어요 = I don’t trust myself
너는 자신을 안 믿어? = You don’t trust yourself?
그는 자신을 안 믿어요 = He doesn’t trust himself
그녀는 자신을 안 믿어요 = She doesn’t trust herself
그들은 자신을 안 믿어요 = They don’t trust themselves
It is common to place the following words before 자신:
나/내/저/제 if the subject is “I”
너/네 if the subject is “you”
그/그녀/자기 if the subject is “him” or “her”
그들 if the subject is “they”
For example:
저는 제 자신을 안 믿어요 = I don’t trust myself
너는 네 자신을 안 믿어? = You don’t trust yourself?
그는 자기 자신을 안 믿어요 = He doesn’t trust himself
그녀는 자기 자신을 안 믿어요 = She doesn’t trust herself
그들은 그들 자신을 안 믿어요 = They don’t trust themselves
Sometimes the person is the subject and also placed before ~에 대해 or ~에게 in a sentence.
For example:
… 자신에 대해 … = about oneself
… 자신에게 … = to oneself
Below are many examples showing the use of 자신:
청소년들은 그들 자신을 사랑하지 않아요
= Young people don’t love themselves
그 학생은 자기 자신에게 자꾸 변명을 해요
= That student always makes excuses to/for himself
저도 제 자신을 몰라요
= Even I don’t know myself
저는 제 자신에게 그런 말을 많이 했어요
= I said that (type of thing) to myself many times
그 남자가 수업 시간 동안 자기 자신에게 말을 해요
= That boy talks to himself during class
저는 한국에서 제 자신에 대해 많이 배웠어요
= I learned a lot about myself in Korea
그는 자기 자신에 대해 아무 것도 알고 있지 않아요
= He doesn’t know anything about himself
see also here