All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Istikhaarah prayer is recommended regarding all worldly permissible matters such as marriage and the like. The result of Istikhaarah is what is achieved after offering it.
As for feeling cheerful or depressed, it can be helpful but it is not the real result of Istikhaarah for this may be a result of previous desires. Also, one may dislike what implies good for him and likes what implies evil for him.
Besides, it is not obligatory to ask the fiancée to offer Istikhaarah; however, if she offers it, then it will be good. What occurs afterwards of getting married or not is what implies good.
To know how to offer Istikhaarah prayer and know its results, kindly refer to Fataawa 122471 and 81434.
Allaah Knows best.