What is the cause of recurrent thoughts of admiring others besides one's fiancé, and what is the way to get rid of them?

These thoughts are often from the whispers of Satan, so do not pay attention to them, and دفع them by seeking refuge with Allah from the accursed Satan. If your thoughts of suicide are due to these ideas, this is strange, for life and religion are more precious than for a Muslim to lose them for such a reason, and suicide leads to the abode of misery. We advise you to hasten to complete the marriage if the young man is of good character and religion, due to the benefits therein. Hastening marriage is particularly emphasized for those who love each other, for "nothing like marriage has been seen for those who love each other." If the marriage does not take place, do not feel that it is the end of life, and entrust your affair to your Lord, and ask Him for a righteous husband, for "And perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah knows, while you know not."