Have you had your iron levels tested? At one point, mine were very low and no matter what I did, I was ALWAYS shattered. They can be dangerous if too low (but I'm sure they'd have picked up on it), it's worth asking anyway.
I know 1st & 3rd tris are buggers for being tired, but it might be worth eating foods with lots of iron in, or speak to you pharmacy to get some iron supplement tablets or something? xx