As a dad with three daughters, I understand how you feel.
You have spent all your time and energy from the year dot until now nurturing your litttle girl, and you have been god-on-earth to her. She has poured all her love and affection into you - with a degree of wrapping-around-the-little-finger - and now, suddenly, all that attention is diverted away from you to some spotty oik wh odesn't even shave yet!
Fear not, it's part of fatherhood. What you have do do is bite your lip,, and never let her see that this new stage of her journey to womanhood is upsetting you.
You will find plenty of times when she is still your 'little girl' - my daughters are 32, 31, and 19, and i still get the 'can you help me?' look or phone call from them!
Once your daughter has got the hang of boyfriends, your relationship with her will stabilise into a much deeper and more meaningful set-up - adult to adult. You can;t be her mate, her best friend, or her boyfriend, but you can be her dad - and she only gets one, and you are it. It;s a relationship that will endure abd deepen as the years go on, so get past this bit, and the rest will be fine.