I've managed to lose weight on clomipramine, which is probably almost as bad as Paxil for weight gain. I find that low intensity cardio doesn't do as much as it used to, ie not on antidepressants, but high intensity interval cardio works great when combined with long slow cardio base training.
I lost about 1-1.5lbs a week when trying to get down to a good running weight last fall through the combination of low carbs and high protein, 5hrs a week of low to moderate intensity cardio (slow running or time on the bike), 2 "speed" workouts with sprints, and then weights sometimes. I would also run a race (5k or 10k) every other weekend. I still never got down to my pre antidepressant weight, even with 3x more training and because of this my race times were slower. I hurt my foot and ended up gaining back 15lbs in 2 months while watching my intake closely and working out as much as possible around it (3-4hrs a week).
In my opinion, as long as you're not grossly obese, it is way more important to your health to be active and fit. Be proud of yourself for increasing the output and besting your old performances regardless of the scale. I've also shifted my goals from trying to run the fastest 5k to trying to finish an ultra (100 miles for example). Concentrating on the quantity will keep your weight down, no matter how much the antidepressants tank our metabolisms.
If you are only doing long slow cardio, try adding in weight sessions and "sprint session" on a stationary bike or running between telephone poles on your road. Also, rather than taking something like phentermine to quench your appetite, maybe add in some Wellbutrin to speed you up and motivate you?