Is a girl considered an adulteress after attempting to establish an illicit relationship and then repenting?

You did well by repenting to Allah and preventing the man from committing the evil act. Anal intercourse is forbidden even between spouses, and what you did is not adultery as long as you prevented him from penetration.

However, if the man was a stranger to you, then you erred by being lenient in going out with him and being alone with him. But if he is your husband by a legal contract, then it is permissible for him to enjoy you, except during menstruation and anal intercourse, while observing the custom regarding delaying consummation.