She is afraid that her marriage may not be successful because she got to know him before marriage

Praise be to Allah.

What is meant by the love that is forbidden before marriage is a relationship between a man and a woman that includes haraam things such as looking, touching, shaking hands, being alone together, romantic words, the woman softening her voice, and so on. There is no doubt that this is haraam because what it involves is haraam. 

For information on the prohibition on the things mentioned above, see the answer to question no. 103044. 

Love may occur without a person choosing that and without doing any haraam things, such as if a woman hears of a man who is of good character and starts liking him. She is not to be blamed for this so long as it is not followed by anything haraam. 

If a man wants to propose to a woman, it is permissible for him to look at her and to meet her in the presence of her mahram. 

No matter what the nature of the love and relationship before marriage, if a person committed any wrong action in that case, then he has to seek forgiveness and repent, for Allah forgives sin, accepts repentance and erases bad deeds. 

If we assume that you overstepped the mark with regard to saying something inappropriate or shaking hands and the like, then all you have to do is ask for forgiveness. Allah has blessed you with marriage to a righteous young man, so praise Allah, may He be exalted, and think positively of Him, and hope for the goodness, blessing and success that He may bestow. 

No one can be certain that the one who falls in love before marriage will not be blessed in his marriage. This is a matter of the unseen that no one knows except Allah. The most that can be said is that haraam love may be a cause of the marriage not being blessed, because a person may be deprived of provision because of a sin that he committed, as the Prophet (blessings and peace of Allah be upon him) said: “…a man may be deprived of provision by a sin that he commits.” Narrated by Ibn Maajah (4022) and classed as hasan by al-Albaani in Saheeh Ibn Maajah. 

But if a person repents and turns to Allah, there is the hope that he will be successful with Allah’s help, and that his bad deeds will be turned into good deeds. 

So have faith in Allah, may He be exalted, and put your fears aside; seek the forgiveness of Allah from all sins, minor and major, and you will be fine, praise be to Allah. 

And Allah knows best.