Is your doctor aware that your current anti depressant doesn't seem to be working? I'd check with him/her to make sure you are on the right prescription and/or dosage.
Many of us have to give ourselves a little push to exercise. The great thing about it is once you're done you normally feel wonderful, and that wonderful feeling is what inspires you to do it again!
I'd suggest starting small and something easy like walking. I splurged ($29) and got an expensive pedometer that's very accurate (cheap pedometers will count steps even when you're not walking
) Sometimes just adding some extra steps to everyday tasks can invigorate and inspire you to do more. LOL sometimes I'll even make a game of it...running up and down the steps to carry extra loads of laundry. It became fun to see how many extra steps I could get in each day. Now I walk on a regular basis, go to the gym, and participate in 5ks.
This may not be your thing it's just a suggestion. Everyone is inspired by something different. You just have to find your inspiration!