Is making a promise to oneself the same as making a vow or oath?

Praise be to Allah.

What is meant by an oath or vow is committing oneself before Allah, may He be exalted, to do something. Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:

A vow is a type of oath, and every vow is an oath. So when the one who makes a vow says: I owe it to Allah to do such and such, it is the same as his saying: I swear by Allah that I will surely do such and such. What is implied by these two phrases is the committing oneself before Allah to do that action". (Majmoo‘ al-Fataawa  35/258).

Another form of oath or vow is making a promise to Allah, may He be exalted, such as saying: I promise Allah that I shall do such and such. Shaykh al-Islam Ibn Taymiyah said:

"Promises and pledges are similar, or the same, in meaning. So if someone says: I promise Allah that I shall do Hajj this year, this is a vow, a pledge and an oath".

See the answer to question no. 38934 .

But making a promise to oneself is not regarded as a kind of vow or oath; Rather it is a statement of one’s resolve and decision to do that deed.

And Allah knows best.