Must a girl who has sinned and repented inform her future husband about what happened, and is it necessary for her to marry the person with whom she sinned, and how can she deal with her pangs of conscience if she does not inform her husband?

You should conceal yourself with what Allah has concealed you with, and do not tell anyone what you have done, in accordance with the Prophet's (peace and blessings of Allah be upon him) saying: "Whoever is afflicted with any of these abominable things, let him conceal himself with the concealment of Allah."

If your husband asks you about the reason for losing your virginity, you can allude that it can be lost for many reasons, without informing him of your sin, unless he stipulated virginity at the time of the contract, in which case he has the option.

You are not obliged to marry that man, but if he repents, there is no harm in that. As for your pangs of conscience, they should disappear with sincere repentance, for Allah has guided you to it, and you should rejoice in that.