How to motivate myself to exercise if I'm severely depressedI'm late to the party but I want to toss in my two cents anyway because from what I've read, I like you already.
I'm not just doling out gratuitous praise. You seem like a very conscientious person who takes responsibility for themselves, and I have immense respect for that. I need to point that out to you because people like you don't give yourselves enough credit. It's so easy for you to find excuses to numb yourself in hypersomnia, binge eating, alcoholism and other self destructive and pernicious activities. But here you are, taking the road less travelled.
Okay, pep talk wasn't what you asked for. Here's the nitty gritty.
- Keep doing what you're already doing. You're seeking professional help, taking medication, duly following the doctor's orders. You're already well on your way to making depression your bitch.
- Find a physical activity you inherently enjoy. I was in a rut a couple years ago and everyday I felt like depression was wringing me in the neck and telling me what to do. Sleep 12 hours a day. Binge on ramen at 2 am. When I decided enough was enough, I took up running. Took up running. I know how comical that must sound to you. One does not simply take up running. Especially when one is depressed. Cue meme. It was a struggle, but I kept it up for a year running five times a week averaging 3-6 miles a day. It was the single most defining activity in my recovery.
...but guess what? I stopped. I don't run regularly anymore. It's a very rewarding activity but at the end of the day running is still something that strains me to have to do. It's something that's only rewarding to me after the fact. Instead, I hike now. I truly enjoy it, no arm-pulling required. When I'm not hiking, I look forward to the next time I get to hike.
Hiking might not be for you, but you need to find a way to exert energy that is intrinsically enjoyable for you. It will take a lot of experimenting, more than you've already done. Venture outside the gym and do activities outdoors. 
The point is that exercising isn't a goal. It needs to be a lifestyle. If you have to beat yourself up to do something, even if you do it consistently for awhile, it will be difficult to maintain. You'll go through cycles of self-loathing when the motivation just isn't there for you to brave through another 8-mile run after an exhausting day at work.
- Be that loser that takes the stairs...and walk everywhere. Forgo the escalator. If you currently drive/transit to work, bike or walk instead. Even if it takes you an hour to get there. It will be easily incorporated into your daily schedule, and it's not like you can just stop and go back. You've already committed to work. You have to commit to getting yourself there.
- Eat good food. I'm not a doctor, I'm a 21-year-old with two letters behind my name. But, a lot of psychiatric disorders can be very effectively treated with the right nutrition that addresses chemical imbalances in the brain. This will not only help you feel better and more motivated, it will literally make it easier to exercise. I don't subscribe to any specific diets. 80% of the time I try to eat a diet consisting of mostly fruits and vegetables (look for variety and vibrant colours) and 20% I eat whatever the hell I want.
It's an uphill battle, I know. You should remind yourself from time to time the progress you have already made and applaud yourself for living life with such purpose and intention. No effort is ever wasted.
 Extrinsic vs. Intrinsic Motivation: What's the Difference?
 Exercising outdoors is two birds with one stone. You are exercising and getting exposure to sunlight--both natural sources of serotonin which improves your overall sense of wellbeing. How to increase serotonin in the human brain without drugs
 The Mood Cure: The 4-Step Program to Take Charge of Your Emotions--Today: Julia Ross: 9780142003640: Amazon.com: Books