I have been on Zoloft for 6 years and have had weight gain (10kgs). My GP confirmed that this quite often could happen when taking antidepressents.
I have just joined Weight Watchers mainly because I need the help and support to lose weight. I will say that my diet hasn't been the healthiest over this period. We are told weight loss is a combination of diet and exercise. (have been with them 3 weeks and lost 1.6 kilos)
As it is difficult for you to exercise perhaps you need to address your diet. They do say that if you don't eat enough of the correct foods, your body goes into "famine mode" and stores fat. If you eat small healthy meals more regularly, this helps increase your metabolism and hopefully address your weight issues.
If you are able, I would suggest consulting health professionals experienced in issues relating to paraplegia i.e. Occupational Therapists, Physio therapists or maybe even exercise physiologists. I am sure they would be able to offer some exercises that are within your abilities.