For LotA there's a grand total of 6 tanks, only 1 of which is MT. You don't have to pick up everything in that case, just try to get as many as possible and concentrate on your adds.

In ST/WoD you have 1 MT and 2 OTs, again, you don't need to pick up everything but there also isn't a lot to pick up. Those flowers in the Cerberus fight can hit like a truck so make sure to pick those up if the other tank isn't.

If you don't have it already, get Provoke. It will make getting aggro back on one mob a lot more manageable. Provoke -> Tomahawk or Provoke -> OP.

If you're hitting more than 1 mob with OP/SC, you shouldn't use Tomahawk. You want to get aggro on as many things as you can in one GCD.

You're probably doing better than you think you are, just try not to panic when things get a bit hectic. You're not the only OT, the other(s) need to do their job as well.