Qitzur Shulchan Arukh – 67:2
Similarly [just as one avoid a neder, a vow about an object], so too one should distance oneself from a shevu’ah [an oath about an action]. However, if he erred and took an oath about something, he should not ask [a sage to find a problem annulling it] but should uphold his shevu’ah — even if it causes pain/hardship. As it says, “he that swears to his own ill, and does not annul” (Tehillim 15:4) and after [the rest of the list of good deeds being praised it continues], “whomever does these will not be removed forever” (v. 5). We do not ask [raising a fault annulling the oath] except in times of duress.
This se’if is pretty straightforward once you know the difference between a neder and a shevu’ah.
A neder applies to an object. I prohibit use of an object, I sanctify it.
A shevu’ah applies to the person; it is an oath to perform or renounce an action.
The gemara that makes this distinction, at the beginning of tractate Nedarim, is the origin of the famous distinction (chaqira) that Brisk uses in many situations — gavra (person) vs. cheftzah (object).
I wrote about this topic, how a neder prohibits something “al nafsho” in a way that a shevu’ah does not and thus how they serve very different roles in one’s avodas Hashem. If you’re interested see here.