I feel you man, I felt the same when I hit 50 and after looking around the net I just had to test out different rotations and this one gave me the best results:
1. Open with Garrote (that equals around 3-4 autoattacks worth of dmg, unmitigated by mob armor)
2. Immediately 1-point SnD (8 seconds should be enough to finish of a mob or get it to ~10%)
(2b. 1-point Rupture if Ruthlessness proccs)
3. Sinister Strike
4. Keep Rupture up
Main point here is that for some reason mobs tend to have 30-40% armor reduction if they're not casters (dunno if it's bug or working as intended). Thus it beats to use Garrote and Rupture over Cheap Shot and Kidney Shot as they don't get mitigated. SnD gives you better chance of getting sword proccs and is best applied at the start.
Best of luck, and don't give up!