Matthew 5:33-37 says,
33 Again, ye have heard that it hath been said by them of old time, Thou shalt not forswear thyself, but shalt perform unto the Lord thine oaths:
1) To "forswear thyself" means to "break your oath, or perjure yourself". I understood "perjury" to mean lying under oath, but I am not a lawyer, so I looked up "perjure" in the dictionary. It means, "to make (oneself) guilty of perjury by deliberately testifying falsely under oath." Therefore, the charge to "not forswear thyself" means literally to not say things "under oath" that you know are not true.
2) The charge to not forswear thyself is followed immediately by a follow-up charge to "perform unto the Lord thine oaths." It is this charge that I have misunderstood until now. I used to think it meant that we are supposed to hold our oaths (promises) sacred and perform them as if they were oaths made to the Lord, but, in the context of these verses, I've changed my mind in a significant way. I now read it as saying that when I make an oath (promise or covenant) with the Lord, I need to make sure I am able to "perform" that oath "unto (Him)". In other words, I can't forswear myself in my covenants with Him.
3) Verses 34-36 provide examples of ways that promises and covenants can be established on illegitimate foundations, then verse 27 lays out the new foundation for covenants and promises. Ultimately, this passage says that I am held accountable for my own actions - that I can't blame God (or the Devil) for anything I do.
4) Finally, my word should be good enough. If someone asks me to do something, I should be able to say, "Yes," or "No," and have that be enough.
That is a goal worth pursuing.