My marriage Istikhara was positive, having been done repeatedly and by multiple sources, so we proceeded to engagement but it broke after a few months. Why was the Istikhara positive and how can we trust this process again?
as salam alaikum
the purpose of istikharah is not to tell what will happen in the future but to indicate the best course of actions to be undertaken by the believer who put his trust in Allah.
One of the underlying notions in the Du'a Istikharah of Imam Zayn al-'Abidin, peace be upon him, is that the servant should be pleased with what Allah has decreed for him. In it there is a huge wisdom that the servant may realize later on, or even after his lifetime.
It is important however to consider istikharah as a du'a and a request to Allah, not a prediction of future events.
With prayers for your success.