How can I forgive a friend who betrayed my trust and secretly contacted my husband online, even though time has passed and she has apologized, and I still suffer from a lack of trust and a feeling of discomfort regarding my husband's relationship with her husband?

The relationship between non-mahram women and men has শরীʿআহ-prescribed limits and controls, and negligence in this regard is contrary to Islamic law. If a woman oversteps the bounds of Islamic law in her relationship with your husband, you must advise her, enjoin good upon her, and forbid evil. If advice does not work, there is no sin in shunning her and avoiding her company, for sometimes a beautiful shunning is better than harmful association, especially if it leads to corruption in religion.

However, if she repents and becomes righteous, one should not dwell on what she did, for "the one who repents from sin is like one who has no sin." And if she has wronged you, it is better to forgive her, for forgiveness increases its owner in honor and dignity, and is a path to attaining Allah's forgiveness and pardon.