What should one do who proposed to a girl and was convinced by her, and his spirits for her acceptance were raised after seeing her, then the girl rejected him after performing Istikhara, while he still feels distressed and sad, and he saw a dream that gives him good tidings of her?

Being keen on marrying a religious woman and performing istikhara are both commendable actions. The essence of istikhara is to entrust one's affairs to Allah, so He may choose what is best for His servant. If the marriage proceeds, it is good; if it does not, then averting it is better, without relying on feelings of comfort or dreams. If you are not destined to marry her, do not lament, but seek another, for Allah is capable of replacing her with someone better for you. As Allah Almighty says: "But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah knows, while you know not." (Al-Baqarah: 216). Strive to repel thoughts of regret by seeking refuge with Allah from Satan and engaging in dhikr (remembrance of Allah), for "Unquestionably, by the remembrance of Allah hearts are assured." (Ar-Ra'd: 28).