I will never get Pregnant! :(

I have been ttc and I have no luck. I have been on metaformin for pcos but also to conceive. I'm 185 lbs and 5'5... and I get my period every month even without metaformin. I know I'm over weight but why can't I get pregnant if I get my period every month. 
It tears me up that I can't conceive but my sisters can and they are younger than me. Each sister has at least 2 and I have none and me being the oldest.