I think for me personally it was a culmination of things and not just one moment. It is different for each person but I think everyone struggles with staying motivated.
There's occasional days where I don't feel like going to the gym or exercising and I'll say to myself "Ok, just put on your gym clothes and see how you feel" then I put them on and, usually, still don't want to go. So then I tell myself "Just drive to the gym and if you don't want to go in you can come back home" so I drive to the gym (10 minutes away) and still don't want to really go in. I tell myself "Go in and spend 30 minutes, then you can go home" so I go in and once I start running I'm in there for over an hour. It's just baby steps and that's what works for me. Not forcing myself to do something I don't want to and that the option is always there to not go forward but once I start going forward, I find, I don't want to go backwards.
Now, the reason WHY I do it in the first place. Several things, I gained 30 lbs. a few years ago when I moved away from home. I felt so miserable and had such low self esteem. When I moved back home and lost more than the 30 lbs. I felt so good about myself, better than I ever thought I would. My Dad had a heart attack a month ago (he is only moderately overweight and 60 years old) and that is what got me seriously motivated to train for a half marathon (I write a "goals" list every year in January, most of those goals get accomplished or started but wanting to train for a half never did until this happened). Now I don't even think about weight loss, when I'm running I think about seeing my Dad having a heart attack in excruciating pain and how I never want to be like that, I think about how amazing it is that just putting one foot in front of the other can completely transform my body, I think about all of the failed relationships/engagements, all of the things I started and never finished, and that pushes me for another mile or for a faster time.
I also take it one day at a time. It took me 2 years to get the 30 lbs. off. Could I have done it faster? Yes. But by slowly changing what I eat and slowly incorporating exercise I truely believe that that is what has kept me on track. Of course, there's days when I don't do as well but I never beat myself up about it. Who cares? Tomorrow's a new day. I hardly ever weighed myself because I didn't put a restriction on when I had to lose the 30 lbs. I just figured if I made little changes along the way and stuck with them then the weight would come off. But you do have to do some research to know what is and isn't good for you to make an informed decision about those changes you need to make.
Another thing that I think really made me believe in myself (and that's all getting healthy is, believeing in yourself) was watching the Biggest Loser last year when the contestants ran/walked the half marathon and the full marathon. I thought to myself, those people are still a lot heavier than me and they're doing it! Why would I ever think I couldn't?!
Part of my "starting slow" included things like "doing 5 push ups a day" or "jogging on the treadmill for 5 minutes". After a week or two I bumped it up to 6 push ups and 6 minutes. I did this every week. I can now comfortably run for 2 hours when I used to be gasping for air after 8 minutes (and I'm up to 50 push ups

). I also changed my diet slowly. Like instead of two scoops of ice cream I'm going to have one. Then the next month I'd try out different frozen yogurts and mix half a scoop with half a scoop of ice cream. Then the next month I would eat only frozen yogurt. Same thing with soda. First I cut back how much I was drinking (only allowing myself one a day), then once a week, then I mixed it little by little with diet, then only diet, and now none at all. It takes time but I find it's a lot easier than doing away with everything you love at once and hitting the gym for 2 hours only to be sore and discouraged.
Good luck! You CAN do it. Everyone is different and I hope you find your moment of "Enough is enough". You deserve it.