This question about what to do when you "love" someone who has a serious relationship with someone else is a complex one.
First of all, it is important whenever we consider relationships with others of the opposite sex to be sure our age and maturity is such that getting involved with someone would be wise. Many teenagers unnecessarily feel the need to "pair off" long before they complete their education and are prepared for married life. The book of Song of Solomon advises us to not stir up love before its time (Song of Solomon 2:7). God clearly wants us to reserve an intimate relationship with another person for marriage. God desires that all young people enjoy and appreciate a mate—but all in due time and when we are best prepared.
When people are ready for marriage and a man and woman are already involved and somewhat committed to each other, the most prudent thing to do is to accept it. As for believing that God wants us to have a relationship with someone instead of another person, this is usually just wishful thinking. God does not normally communicate with people about such matters. Rather, our own human emotions and feelings get involved and we convince ourselves that God wants us to "have" a particular person.
One thing we can know is that God would not have us break up another couple's relationship. That is in the hands of those two people. God gives us all free moral agency and desires that we make wise decisions about future mates—but He wants us to consider His Word in doing so. Galatians 6:7 tells us that we will reap what we sow and in 1 Thessalonians 4:1-7, God tells us to conduct ourselves in an honorable and holy way in courtship.