(Presuming you are male based off your user name.)
If it were you engaged to the lady in question, how would you feel if somebody were actively praying for your engagement to break up so he could marry her instead?
You wouldn’t like it much, would you? It would probably strike you as mean and selfish, and you’d be hurt and angry, wouldn’t you?
What scenario are you imagining, exactly? That you pray to God that the lady will suddenly have an epiphany and realize that you are her One True Love and she dumps the man that she’s spent a lot of time building a relationship with? You realize that only happens in romantic comedies, yes? (Which mysteriously tend to downplay how hurt the dumped party will be as a result of this?)
Or that the man will dump her, leaving her free for you, and your plan to be waiting by when she gets dumped, and declare how much you’ve always loved her, and for her to fall into your arms? Again, you do realize that only happens in the movies? In reality, she’s likely to be heartbroken, and need time to heal, and even when she does….there is no guarantee she would choose you.
Praying to God that she’ll cast her eyes your way isn’t love. It’s objectifying her. She’s not a person to you, she’s something to be obtained, no matter who gets hurt to grant your desire.
Did you ever have a chance with her when she was single? Yes? Then why didn’t it work out? No? Again, why didn’t it work out?
Did you meet her after she was already in a serious relationship? Then sorry, but the fates were already sending you a sign that she wasn’t the one for you. Take what you admire about her and look for a single woman with the same qualities—obviously these are what attract you.