5.7 does not seem that short to me, lots of men that size not too mention women.
YOU should calculate your "ape index", I would think that the higher your ape index the lower your spl which is not necessarily the best match for your O2 needs.
I'm a beginner (ish almost two years in learning by myself) I don't spend that much time swimming freestyle though I read quite some things on the topic as well as watch quite some vids (not too mention others swimmers).
I don't pretend to do it right (various issue and whereas I work on proprioception it is still far from perfect) the most common issues I see with other swimmers are:
Bad overall body alignment /streamline
Bad spearing /extension (from over crossing from having the forearm moving upward)
Kicking too hard
(there is breathing but some open water swimmers purposefully have a weird breathing technique).
Too high SPL, I see a lot of people putting a F lots of efforts in the water and not maximizing their work.
All that said some are still going pretty fast, there is hope for all us ;) Definitely champion elite swimmers are tall (especially freestylers and basck strokers) but it is not what is holding you back.
When I feel down with my progress the thing keeps me going is the "feel" of water, experimenting with water reactionto pressures, working on proprioception in the water. Swimming in another person turbulences (and make it into a smooth ride), etc.
Putting the chronometers down 10 minutes (or more) and rediscovering "water", keep the experience fresh, an on going discovery of the medium which we are travelling through: time... I mean water :=)