If you are holding them off 1v2 at your tower, that means your team should have either a 4v3 at top tower OR should have a free push on mid with equal odds at top. Either way, as you were bottom and have a long haul to get there, you should have been the LAST person needed to make the haul over there. Your trundle should have been heading that way once pings came in and as mid was empty (3 top and 2 bot), mid needs to make the call to either free farm/push while they can or swing top to balance things out.
I'd also like to know how things went down up top. Janna/Taric is a strong team for defending a tower. You've got Janna's shield for the tower, her ult if things get dicey (assuming she was 6), and the rest of her soft CC, + Tarics heal and stun. That's an awful lot of kiting you can do around the tower even 2v3. In any event, if the tower was lost if they die, then they should have retreated before being killed. Backing down and losing a tower is bad, but dieing and losing the tower anyways is worse.
It honestly sounds like there were plenty of mistakes made among your team, but I don't think your decision to stay bottom (assuming your recollection of the events is accurate) was one of them.