second best or simply new found soul mate?

hiya well what happened was before i even got together with my boyfriend, he had liked my mate and asked her out. she turned him down as she didnt like him in the same way. they don't keep in contact now, however, quite a while later we began talking - me and my -soon- to -be boyfriend and he asked me out. i have now been going out with him for a couple of weeks, but cant help wondering if he would prefer to be with my mate than me, i feel so depressed over this, please help. how could i get over this any advice? i'm not sure why i think this as he doesnt even talk to her anymore, but i cant help picturing him and my mate being a couple, and feeling so low. please help thanks xx