I have a friend who likes my brother a lot. She has been asking me to introduce her to him for a long time and the only reason I haven’t is because I know my brother doesn’t take relationships very seriously. He will go on a few dates with her, she will think they are together, and he will then dump her for someone else without giving her a reason. He does this all the time and has been doing it for years, and all I want to do is protect her from this because I don’t want it to affect our friendship. I have tried giving her my reasons multiple times, but she just doesn’t accept it. This is starting to create a lot of tension between us because she has this wrong idea that I don’t think she’s worthy of going out with my brother. What should I do?
If this is causing friction between you two, your friendship will be affected irrespective of whether you introduce them or not. As adults, we all have to learn lessons on our own. You can try and protect her, and your heart may be in the right place, but maybe the only way for her to put this behind her is to try and get to know your brother better. Maybe all she wants is to be able to make up her own mind, without your intervention. If this affects your friendship, all you can do is hope that she remembers you trying to protect her.

What can I do to convince my girlfriend to stop hanging out with a few people who are a bad influence on her? I don’t want to come across as someone trying to control every aspect of her life.
You can give her your reasons and hope that she sees things from your point of view. Beyond that, whom she chooses to be friends with is her prerogative. If your reasons are compelling enough, she may come around to accepting your suggestions.