First you must determine where the forces are coming from that are causing
this relapse.  Often, people with ortho relapse will have a night-time habit
of moving their jaws a specific way while clenched together.

Johanna-

Another treatment could very well work.  The biggest problem with
orthodontic treatment is retention, and so the issue probably lies
more in what's going on after the braces come off, than whether or not
they did their job properly while they were on.
I don't know anything detailed about your case, but I see a lot of
patients at the practice I work for who have bonded lingual retainers
on their lower (and sometimes upper) front few teeth.  This prevents
them from slipping past each other and getting out of place from
normal jaw function.  You might consider having one of these
'permanent' retainers, in addition to a Hawley (the pink plastic/wire)
retainer.  Hawley retainers do allow your teeth to settle vertically,
however, and it seems that this vertical settling is actually not
helpful to your situation; you're having too much of it.
It is regrettable that your essix retainers (the clear ones) cracked.
Usually, these retainers are excellent at keeping teeth where they
belong, and do not allow too much vertical settling of the teeth, as
they cover the entire surface of your teeth.You also mentioned wearing
your night guard; if you are referring to a positioner (they look like
mouthguards with your teeth imprinted in them) these are excellent
options too.  Once you have completed your second round of treatment,
if your ortho thinks it is necessary or appropriate, you could use a
positioner to keep your bite in place and open, the way it belongs,
either instead of or in conjunction with an essix or hawley retainer.

That was a long response! But I do think it is in your best interest
to partake in another round of orthodontic treatment, but if the
results from your first orthodontist did not remain to your liking, I
might advise you to find another one.

-Ashley