It looked like you might have a rabbit strip in the tail. If so, that's a big addition of weight. They move wonderfully in the water but soak up quite a bit and make it heavy to cast.

If I was seeing things and there's no rabbit/zonker in there, then try trimming the deer hair head smaller. That fly has a dahlberg diver style head and weighted eyes, which counteract each other: the eyes want to sink, and the deer hair wants to float. What is produces is a fly that likely sinks a bit and pushes A TON of water. (pushes water- creates vibrations underwater that get the attention of the fish via its lateral line and can work really well when its dark or murky) Trimming the smaller part of the deer hair head down and shrinking the size of the collar will make it much less air resistant and easier to cast on a 5wt. This will also make it run a bit deeper, so keep that in mind.

And judging by the picture, and using the 6"x9" paperback I've got sitting here as a reference, that fly is WAY too big to cast well on a 5wt. Try making it 2/3's the size of that one: both length and diameter.