An internal locus attributes results to your choices; an external locus attributes them to circumstance. Neither is correct on its own in a domain with real randomness.
Traders fail from both ends. A purely external view produces helplessness and conspiracy: the market is rigged, the brokers hunt stops, nothing can be done. A purely internal view produces the illusion-of-control and self-punishment over outcomes that no process could have avoided.
The workable position is precise rather than balanced. Own the process completely - entries, size, stops, whether you traded at all - and hold the individual outcome lightly. That split is the whole of process-over-outcome stated in psychological terms.
Related: illusion-of-control, process-over-outcome, self-attribution-bias, self-efficacy