Bismillahir Rahmaanir Raheem
Al Jawaab Billahit-Tawfeeq
This is an invalid oath. You can only take oath on the name of Allah. Islam prohibits taking oath on names other than Allah's.
Your fiancée (or the woman you are intending to propose) and will not become Haram for you. However, you should repent for the sin as commiting a sin is sinful regardless of whether you take an oath or not.
See - www.muftisays.com/qa/oaths-vows/4501-i-swore-on-the-name-...
And Allah knows best