Can't get pregnant

Okay, me and my boyfriend been having unprotected sex and he inject inside me everytime and I just can't get pregnant I don't know if its because he 16 years older then me or because he smoke pot and pop X pill (he already has 2 kids prior to me) or its just me that can't have kids because I been on depo before and been off for like 5 months . help please it just hurts me I can't give him the child he wants💔