The reason why men don’t respect nice girls is that guys always see nice girls as dumb. Nice girls are not dumb but when a nice girl gets into a relationship, she regresses. She becomes who she really is. What happens is that you don’t lose your intelligence but rather you lose your emotional intelligence and social intelligence. 

First understand that social intelligence is knowing that the way you see people is tinted through your insecurities and nice girls focus on what people did to them rather than what they did wrong to get those types of results. So you gotta get out of that mindset of constantly thinking of yourself. Start seeing people for who they are, consistently keep updating people’s perception and if they are not good for you, leave them.