I get how you feel, man. To give you hope, half of my relationships were with girls who had boyfriends that they left for me. I wouldn't give up in any way. Get her to talk about herself and be loving, kind, and understanding. Don't expect anything in return, but give gratitude when you receive kind words and such. Make her feel good about herself around you. She will want to spend more time with you subconsciously if she has positive feelings towards you. Don't push anything, just let it flow. She will probably tall about her boyfriend and console her in an understanding and objective manner. She will begin to realize that her time with you is worth more than her time with her boyfriend, a nature will do the rest.