Hi EMZ, and thank you for your translation.
I think there are some errors due to misinterpretation that I want to point out. For instance, I think the verbs in future tense should be translated that way, like:
1. First stanza - "tomorrow you go" -->"tomorrow you will go".
2. Second stanza - "llorarás" (and repetitions) --> "you will cry".
The whole verse would be, "You will cry, you will cry due to /because of your whim". (Something like that)
3. Second verse, "I know it is me you who you want', I think it should be "I know it is me whom/the one you love". In Spanish, in this context, "querer" is "to love".
4. Also, in the first stanza, the word "solo" you can translate as "single", since she is going to marry somebody else but, actually, it means "alone".

Let me ask you something about English:
In the first stanza, do you need "to" all those times? If the title is "I have promised you", I would say the same in the first verse, and I would finish it, "...I have to forget you"; "I gave you" (?)
In the second stanza, "you could not be happy with nobody else", shouldn't "nobody" be "anybody"?
I would appreciate your answers.

Best regards