Hold it!  What do you mean "When I gain it back"??!!!   This journey is a
mental one too, not just a physical one, and thinking/writing/saying things
like that is self-sabotauge.  How about this instead?  "If I start to gain
weight back for some reason, my clothes will start getting tighter, and I
will notice it, and either cut back on my eating or start exercising more."  
Seriously, say goodbye to those larger sizes FOR GOOD.  

There's no shame involved.  We need to get rid of shame and look at this
dispassionately: it's just a problem.  Life has all sorts of problems,
that's sort of the whole point of being alive.  The bright side is, and
this is my motivation, is that the particular problem of being overweight
is (in at least 99% of cases) totally solvable!!  It's hard work, yes, but
it's possible, and worth doing. My motivation for doing this is that it's
my job as a person to be the best version of myself that I can, which means
solving the problems I can and not stressing about the ones I can't. It's
about the whole mental outlook of seeing every problem as a challenge or an
adventure, not whining about it, but using my brain and my skills to find
solutions and implement them.  I'm sure there are challenges in your life
or your job that seem daunting at first (big project at work, paper due at
school, closet a disorganized mess) and there has to be _something_ that
you do in your life where you actually find joy and satisfaction in
figuring out how to approach the problem/challenge and then conquering it.  
Heck, it may even be something as inconsequential as doing a jigsaw puzzle.  
Whatever it is for you, try to apply that same mental outlook to weight
loss. I had to train myself to do this when it comes to cleaning: I love
sewing and art projects and decorating, but I used to hate cleaning, so I
made myself look at it as just another way to beautify my surroundings, and
then it became much more pleasurable.  We can all do the same with weight
loss, find something about the process, not just the goal, that makes it
enjoyable and heck, sometimes even fun.  

Sorry for that: shelf...