I am single, female, 33y.o. virgin and have recently found myself having very vivid inappropriate thoughts about my older brother's (unmarried) best friend. Should I tell him that I have been lusting after him and ask for his forgiveness?
No, you should not tell him you have been lusting after him, nor do you require his forgiveness. If you thought to steal a candy from a store, do you owe the store restitution? No, for nothing was truly stolen.
It is illegal desire, and for that you sin against yourself and God. Lusting is wrong because that man very well may be someone else’s husband one day and not your own.
It is okay to have sexual desire, or if you look upon someone and find yourself aroused, this is natural (I don’t mean to stare and reach arousal). But God made us to desire the opposite sex. This is healthy. Where we err is if we fantasize about them in a sexual manner or as your vivid imagination experienced, you play out scenarios in your mind. You in essence fornicate in your mind (the act of having sex outside of marriage) with someone you are not wed to.
Intimacy is a beautiful thing God made for a man and woman within the marriage covenant. It is something to look forward to, and it is natural to desire it. There is no shame in it. The opposite sex is attractive.
As best as you can, and depending upon the grace of God by laying your temptations before Him by approaching the throne of grace in the time of need, let the Lord provide the grace and mercy you need to practice self control and escape temptation.
And remember, the Lord forgives you. He doesn’t condemn you. He loves you. You are a daughter of God, cherished and loved. You are of value. Lay your desires for a husband before God, and ask that He would provide in that area of your life.
God bless and may the Lord strengthen you with righteous convictions.