If you are losing post laning phase with a tank, maybe you aren't focusing team fights? If you play someone like Mundo, you can gank mid or scuttle/enemy jg very easily with Q. Perhaps try leaving lane before your team shits the bed.

If you want to be innately useful to your team, play Ornn. He's got good CC, extra resistances from passive, can stay in lane forever, and he can upgrade mythics.

If you're having trouble against champions like Fiora, try playing Kled. I love playing Kled against aggressive champs like Fiora or Renekton. He's built to run at the enemy, and since they added grievous on his Q, he's become great against the plethora of healers in top.