Cheers
Don't worry. It will happen
Unless you're a machine of sorts I guess. Don't be afraid to put in more than one medic. For some bosses I even used three just to keep up with the abuse that was being hurled my way. Of course that is slightly risky in itself as some bosses will cast Doom on you if you take over 20 minuets to kill them! Its probably always better to have two medics and one dps or two medics and one buffer.
Forget two medics and one sent, that's pretty much hardly ever needed.
As usual there's a lot to do end game so no worries there. You can return to most areas and the more you do of the endgame the easier it becomes to travel around.