Dont Know What To Do : / Need Helppp Please !

Hi my name is nayna, well i need an advice, i really dont know what to do, long story, short i have a best friend that we have been friends since 3yrs n well his really a flirt guy, even since ive been friends wit him its like his from girl to girl and so it got to a point where he was really in love with a girl for 5 yrs now n so that same girl is not in love with him like she use to so his highly upset and well ive been there for him in everything seriously ! so time pass and well his mom knows me and she thinks that his son should be with me, that she will be happy if we be together and get marry but the problem is that we made a promise like 2yrs ago saying that we promise not to fall in love with each other at all so okay we promise that and he told his mom about that promise and she said that love can break that promise and yea i think it does, its just the way he is, like i sayed before he likes to flirt around, so 2 days ago he tells me all about what his mom to told him and that he will marry me in a heart beat so i told him that he wouldnt and he kept saying that he will, so he asked me if i would marry him and i answered yea i would, for the fact that your the only dude i have love for, your my best friend so he got shocked when i sayed yea, so he told me he would marry me when i finish school which i finish next yr, and i told him that he likes to flirt alot so how would i know you will be faithful to me, he said that he will prove it in anyway that he will be faithful to me, so im watching out for that but theres just one thing he told me that made me think alot, he told me that this Saturday he is going to a party to meet his ex girl the one he has been in love with, so they could end everything face to face, okay i really dont believe that, they probably gonna get back together i mean who knows, so im see what happpens from there and on, also its like we know that if we do get to marry people we know will be shocked and amazed about it, things will change i dont know if it will be good or bad, but all i know is that God knows my future and He knows if my best friend is for me or not, i mean if his for me, its like okay good, great just show me you love me and will be faithful to me, and if not then you will always be my best friend and will always love you! but i just dont know what to do about this problem :/ i need helppppp! like asap please !!!!