I have two (depending on how you count, four) major motivators ...
One, I too went through a fairly devastating loss in late 2009. In early 2010, still grieving, I went back to my bike as a means of coping and by and large have never looked back. I dropped 40 pounds, have gained about 16 back, but I'm back on the bike after a hiatus. It's helped me physically, but also mentally, just as much.
Two (and three and four) ... I have three kids. I don't desire to be the "fat dad" who doesn't do anything fun with the kids, or worse, can't because I'm fat. And I want to live long enough to see their kids and maybe even see their kids have kids (a stretch I know).
You can do this ... keep at it, never give up