'Believe in yourself' — as an instruction. You're telling someone else.

What a Japanese person actually thinks
Reads like a message on a sports-drink poster. Perfectly natural Japanese, slightly odd as a self-description — you've tattooed a pep talk aimed outward.

Meaning

Literally: yourself + (object) + believe (imperative)
Actually: Grammatically it's a command or encouragement aimed at a listener. On your own body it reads as advice you're giving to whoever is looking at you.

Scores

Naturalness●●●●●

Completely natural as encouragement to someone else.

Coolness●●●●●

Six characters, mixed script, imperative mood. Everything about it is aimed at a reader rather than the wearer.

Watch out

mediumThe て form here is directive. If you meant 'I believe in myself', that's 自分を信じる — a different ending entirely.

Get this instead

自信 jishin
self-confidence

Two characters for the quality itself, no grammar and no audience.
自負 jifu
quiet pride in one's own ability

Confidence you hold rather than announce. More adult, and rarely seen.

On skin

自分を信じて

Orientation: vertical

Mincho.