I have a problem

Before I get started, let me give you all a bit of history...

I come from an Italian family, at least on my mother's side. I grew up with my grandmother, and all of the women in my family are, well, to put it bluntly: short and fat. There has never been a time in my life when I remember being thin. There were times when I was less fat than before, but I've never been THIN. My mother and grandmother both were the sort of women who would always make you finish your plate, even though they gave you more than you probably should eat, and they were always telling me to eat more, because I looked ill. Oddly enough, I was a rather sickly child, and that was often their excuse. In the beginning, feeding me was a necessity, I was sick all the time and never hungry.
When I got older, and I was sick less often, the weight stuck. My eating habits, unfortunately, did too. I rarely ate -- once a day, maybe twice at most. I should also point out that I've always been uncomfortable with other people, and went out of my way to expressly avoid changing during P.E. classes at school, or even participating in team sports.
As I got older, the weight didn't really go anywhere. I didn't gain very much, but I didn't lose much either.
Then, when I was 19 or 20, my boyfriend of the time, his father, and I were out celebrating my boyfriend's birthday. It was after dinner, and we were on our way to go get some coffee before we took his father back to the train station so he could head back to NYC. It was a cold and rainy December night, and a woman in a black PT Cruiser pulled out in front of our poor little white Ford Taurus. Later the woman claimed she thought we were turning. We went from the left most lane across three lanes of traffic into the right side shoulder. Our car was totaled. My boyfriend had broken his thumb when the airbag burst. I was covered in fettuccine Alfredo, but I felt fine at first. An off-duty police officer two cars back called in the emergency, and had seen the entire thing (it wasn't our fault). By the time the ambulance arrived, I realized that I was in an immense amount of pain. It was starting to become very apparent that I had busted my right leg -- I had seen the woman pulling out in front of us, and instinctively put my foot down on the floorboard as if I were hitting the brakes on the car. I was in the passenger's seat. By the time I got to the hospital, even flexing my toes became excruciatingly agonizing. I was told it was simply a contusion, and to go home. No crutches, no pain relief, no xrays or scans of any sort. Just a man moving my leg, and me screaming in a hallway after having to lay there for three hours.
It took me another two hours to manage my way up three flights up stairs because of the pain, and another six months before I could walk without my knee buckling. During that time, my weight just skyrocketed. Before I knew it, I had breached the 300lb marker.
When my boyfriend and I split up, I moved in with my mother in Florida, and there I lost a bit of weight, getting back down to something more manageable, just over 200. When I came back north to Delaware, things just got super stressful, and I think that stress helped me with gaining all the weight back. It's been an up and down ride since that point. I've found out that the cartilage in my knee is completely gone, I've undergone several visits to an orthopedic doctor, and the best they can suggest is physical therapy. It's nearly impossible for me to move comfortably on most days because my knees are completely shot and full of arthritis.
When my current boyfriend and I moved into the apartment we currently are in, we eventually bought an exercise bike - he picked it out specifically to be gentle on my knees.

I joined this site at the end of 2010, and yet I've gone up and down in how much weight I've lost. I'm STILL over 300 -- hovering around the 340 range, and I can't seem to get the motivation together to exercise. I want to be healthy and I want to lose the weight -- so I feel better, so my knees have less stress on them, so that I can wear cute clothes. Why can't I just DO it?
When it comes down to it, why do I always put it off?

My boyfriend is a healthy eater, and he exercises on a regular basis -- he goes rock climbing three times a week for HOURS and comes home happy and exhausted.

I know that if I were to exercise on a regular basis I'd start losing weight and I'd start feeling better -- I know because I was doing it when we first got together. I was doing zumba and walking and doing push-ups and crunches. My sleeping schedule was starting to resemble something normal, and I was eating three times a day -- though my boyfriend wanted it to be five -- instead of once or not at all.

I don't understand why it's so difficult for me to just knuckle down and do it. I've done it before? I only stopped because I got really sick with bronchitis for about two months and couldn't do more than complain and sleep.

I know that I can get back to that, I know that I can lose the weight, I know that I CAN DO IT. But.... getting started is the hardest part. When my boyfriend reminds me to do exercises, I feel like he's harping on me. But without that reminder, I'm afraid that I won't do it -- and setting an alarm on my phone just doesn't seem to cut it.

I have a hard enough time remembering to log in to MFP every day and log my food and exercise when I DO manage to get any done.

TL;DR end of it: Suggestions for motivational reminders to exercise and eat regularly (instead of not at all)?