What should I do about returning to my ex-fiancée, who saw me with her in Paradise during Salat al-Istikhara?

If the girl is religious and of good character, there is no impediment to re-proposing to her and solemnizing the marriage contract with her if both parties desire to marry. It is prescribed for those who wish to marry to perform Istikhara (prayer for guidance), mentioning the other party by name. If the matter is facilitated, it is good; and if an obstacle is found, that is a diversion from Allah. Istikhara does not rely on visions.