Another endorsement of the above recommendations, with a few nuances. Shorten your leader to 3' until you get the feel. Lighten your fly (as per Ralph H) and have either a full sinking line or a Type VI sinktip to get it down.
The cast is a one-and-done. Roll cast to get your sinking line to the top. Immediately start your backcast, using the water tension of the sinking line (up top) to load the rod (a "water haul"). Keep the back cast high and then haul once (pull down with line hand) as the forward cast develops. Keep an open loop, release high and shoot the line.
Using this approach, I can cast 60' with a full sinking line with maybe 25' of line out of the rod when I launch my back cast: the rest is on the shoot.
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The alternative is a floating line, 12' leader and heavy fly. It's an SOB to cast and there's a danger that fly ends up in your back or head, or, worse still, hits your rod and damages/breaks it.