Qitzur Shulchan Arukh – 67:4
If his intent is to establish for himself some [program of learning] in Torah or to do some mitzvah, and he is afraid that maybe he will falter afterward, or he is afraid the [evil] inclination will lead him astray to do some prohibition or stop him from doing some mitzvah, he is permitted to motivate his soul with a neder or shevu’ah. As Rav said, “Where to se know that one may swear to do a mitvah to motivate himself even though he already was sworn and stood at Mount Sinai? Since it says ‘I swore and I commit to keep Your righteous laws.'”
Even if he dd not say them in the language of a neder or shevu’ah, just mundane speech, it is a neder and he is obligated to fulfill it. Therefore, a person must be careful when he says that he would do any act of mitzvah that he say “without a neder“. It is a good for a person to habituate himself thus, even when he is saying to do something of his own will, so that he shouldn’t stumble, G-d forbid, in the sin of [broken] oaths.