Let me see you are 19 trying to get pregnant with a man who is 21 and had "kids years ago" who obviously does not visit his kids, all his kids or is not a dad to his kids.
So is there something wrong with you,? yes, good judgement in my opinion. Bad father material. and too young are good starters.
But it can take couples sometimes longer. Are you actually tracking your ovlation with a tracking kit, or using body temp? ( not online estimates) So you are having sex on the right days. and no on less productivie days.
Have you both had exam's to check ?
Does he wear real tight underwear, this can hurt his production, does he get into hot whirlpools, this kills current active sperm ( will not harm the replacements) Does he masterbate too frequently ?