What was my trigger to finally lose weight?

There really was no single thing that finally made me decide to lose weight, no particular lightbulb moment.

I actually built up to it for six months or more. Over that time, I think I took many small steps toward the final decision to change my life. There were lots of contributing factors, some of which were:
I was choosing what activities I joined in based on my fitness. If I knew I would have to walk a long way, or climb a hill or stairs, I wouldn't go. I was missing out on things because I didn't want people to see how badly my weight was affecting me.
I was careful about what I ate in front of other people - trying not to give them an excuse to laugh at the fat lady eating. This lead to me eating in private what I was denying myself in public - and more besides.
My clothing size was still creeping up. When I shopped at my favourite big-girl stores, I was at the top of the size range and starting to wonder what would happen when I was too big for their clothes. Kaftans, perhaps?
I didn't fit - at least not comfortably - in things like airplane seats, conference seating and other small spaces. I also felt I didn't fit in many social situations.

People often ask me why I decided to lose weight. I actually avoid answering that question when I can. It would be nice to say lots of profound things about my health and wanting to live longer.

The truth does contain elements of those things, but there is a healthy dollop of vanity in there too. I wanted to look better, I wanted to not feel unhappy when I looked in the mirror, I wanted to look my age or younger, not older.

I call it 'healthy' vanity, because in this case it was. It is very easy for vanity to become unhealthy - we just need to look at all the superthin movie stars to know that.

By the time my 40th birthday come around, the decision to change my life was basically made, and just waiting to be put into action.