Motivation is only one part of the equation, medications of any type (especially anti-depressants though) can cause you to gain weight, then there's your life - family, work, finances & whatever has caused you to be on anti-depressants all this goes to making it easier or harder to get motivated.
Depending on why you are on meds, I would suggest that your best way to lose weight is to work on your depression>>>>> reduce this>>>off mmeds>>>more energy & happier>>>exercise will be easier as will eating right etc >>>loss of weight!!
Suggestions on how to reduce depression; try to take a 30 minute walk around dawn (or as close as you can) as there is something in the suns rays at this hour(even when overcast) that works in tandem with exercise to balance the serotonin levels in your brain & reduce stress. Along with this you could take supplements such as sam-e, dhea, vitamin b, st johns wort, valerian, (all reduce stress - but do not take the all!!!!) also 5htp (tryptophan) which is what our body converts tryptophan in foods to & then it goes to our brain to balance serotonin (but if our stress levels overload this system can in effect fail).
Another natural way to reduce stress, anxiety, depression, sleep problems, etc etc is to use a ces machine or sound/light machine (check out dynamind.Com). Remember too that if you slip back one day there's tomorrow don't be so hard on yourself that you just get more depressed.