So long as you have prayed istikhaarah and asked Allaah for guidance, and it has not been made easy for you to marry this girl, then you should realize that this is what is best for you and for her right now. It may be that marrying her would lead to bad consequences, which Allaah has averted from you; and it may be that marrying someone else may be better for you. Allaah grants provision to whom He wills and decrees, and He knows best what is in the best interests of His slave and will bring him happiness. All the slave has to do is entrust his affairs to Him and accept His choice, and believe in His will and decree and hope for that which is with Him, for He is more merciful towards him than he is to himself, may He be glorified and exalted.
Ibn Mas’ood (may Allaah be pleased with him) said: A person may hope for some matter of trade or position of authority, until he is close to attaining it, then Allaah looks at him and says to His angels: ‘Divert it from him, for if he attains it I will send him to Hell.’ So Allaah diverts it from him and he keeps regarding that as something bad, saying ‘So and so beat me to it, So and so outwitted me,’ when in fact it is a blessing from Allaah. Quoted by Ibn Rajab in Jaami’ al-‘Uloom wa’l-Hukam (2/470).
As for your feelings that this girl will be yours, this is possible; it may happen or it may not. You do not know what is best for you, so you should not feel attached to anything. Rather you should ask Allaah to make good easy for you wherever it is, and to divert evil from you wherever it is. Hasten to look for a wife who is of good character or religiously committed, and do not delay marriage in the hope of attaining this possibility, for that is the way of those who are weak in faith and do not put their trust fully in Allaah; one of them may set his heart on something and never be able to forget it, whilst claiming that he has prayed istikhaarah and asked Allaah for guidance, and that he accepts His decree, where in fact he will only be pleased with his whims and desires and what his heart longs for.
To conclude, you should do the following:
1- Ask Allaah to make it easy for you to marry a righteous woman who will help you in this world and in the Hereafter.
2- Strive to look for this righteous woman.
3- Forget about this girl, and stop thinking about her altogether, and believe that goodness is in whatever Allaah decrees for you.
4- Do not feel upset because you were rejected for no reason; there may be reasons known only to her, that you know nothing about.
We ask Allaah to help us and you to do what which He loves and is pleased with.
And Allaah knows best.