What are her reservations? Is it self-esteem/body image based? Jealousy or worry that she won't enjoy sharing you? Legitimate lack of interest in multiple partners?
You have to address her particular concern(s). Are you 100% sure she's only hesitant and not flat out not interested?
Your profile has been around for a while... Did you make it in your previous relationship or when you were single or have you been trying to convince your current partner for this long?