The thing with emotions is that you control them. You can’t help the way you feel about something. Even when you can logically understand something and tell yourself reasons why you should or should not feel a certain way.
When it comes to developing feelings for a girl that has a boyfriend, clearly this is not an ideal situation, but what to do about it? How can you get this girl to like you?
This is a controversial topic and, to be clear, I am not recommending you go around home-wrecking so to speak. However, I still think this is a topic that is worth exploring and considering from a non-invasive perspective.
If you are truly invested in this person and are convinced you guys would be good together, the best thing you can do is let it be her idea, not yours. Humans are very predictable, they need things to be their own idea or they are significantly less likely to adopt and implement it.
So… how can you make a girl that has a boyfriend decide she wants you? With patience, slowly insert yourself into her life. Become a source of certainty, trust, and enjoyment. Be someone she can go to with both her problems and her celebrations. Make it clear you understand her for who she is on a deep level. Listen to what she has to say and use your strengths to add something to her life she otherwise would be lacking.
There is no guarantee this will work, but it is the most ethical and genuine way to approach this question. If things are not meant to work out between the girl you like and her boyfriend, eventually they won’t. If they are, and you actually care about this person, then you must conduct yourself with respect for her feelings and life and move on. Eventually you will.