Height has little to do with it.
In practice something that will help is to move the hurdles a little closer together to make 3-stepping feel more natural. Move them further apart as you go. In practice, if the only way for you to three step is to overstride, even if you don't feel fast doing it go ahead and do it that way. (but if competition, don't, 4-stepping would be better for you until you get it down) Just don't think you can get away with 4-stepping for long, you will never compete with the best girls around you 4-stepping.
As for actualy going over the hurdle, don't be afraid to hurdle a little further from the hurdle than you are used to. Coaches say it all the time "attack" the hurdle. If there is a battle between you and the hurdle, you will win

, so don't be afraid to attack it.
back to the height thing. Look at Gail D., arguably the best female (short sprint) hurdler EVER; she's 5'3". We have a couple of guys at my school 6'0 - 6'2, they have the same problem as you do, they can 3 step one or two (or four) in the race but can't really be consistent with it, whereas I'm only 5'7 and have been three-stepping since I was a frosh (when I was 5'6). I'm not trying to toot my horn but my point was, if you are short, just don't let it get to your head.