Kim-

Do wear it, as long as you can fit it onto your teeth.  If you feel
that you are really having to force the retainer onto your teeth, and
it still can sort of rock back and forth at different points while
it's on your teeth, that's a sign that your teeth are too far moved to
really be wearing that retainer.
The best idea would be to return to your orthodontist, who can make
you a new retainer to fit your teeth perfectly as they are now.  Most
will not charge you for this; some will, and depending on their rates
it can cost a couple hundred dollars to get a new set.  However,
usually at no extra charge, they can make you splints.  Splints are
clear plastic retainers which fit your teeth like the one you have is
supposed to, except they have extra plastic built up on the chewing
surfaces of the teeth, so that you can grind away and not wear through
the retainer and damage your teeth.  This is your best bet, and what
is most often done for patients with bruxism (teeth grinding).  The
ortho will just have you bite down on some colored paper to see where
your grinding occurs the most, and build up the retainer accordingly.
Really, getting a splint for your grinding is the best bet.  It will
relax your jaw, and prevent you from damaging your teeth, and grinding
through more regular clear retainers than you'll be able to count.
And your ortho visit will only be maybe 15 minutes, and your retainer
can be ready in two weeks or less.
Once you have finished treatment, you are never really finished with
the orthodontist! If you went to a good one, they will be happy to see
you again with any problems you may have.

Ashley