Left Headlight keeps turning off

If you have HID headlights, that is the normal failure mode, install a new bulb.

Philips D2R Xenon HID Headlight Bulb,Pack of 1 : Amazon.com : Automotive

Here is a guide (2004 and 2005 have D2R bulbs, 2006 to 2009 have D4R Bulbs, that is why you are not covered)

Someone with small hands has quite an advantage:

 

If you've replaced them w/genuine quality D2R bulbs, then it's possible there's a ballast, wiring or other electrical problem.

You should be glad that your lights lasted as long as they did. I noted how long my D4Rs (used in 06-09 Priuses) lasted at 04-05 (NOT 06-09) Prius owners w/HIDs - how long did they last? | PriusChat.

 

You could swap the lights between the two sides to figure out if it's the bulb or the ballast/wiring for that side.

When did you last have the bulbs replaced? Which side or both?

Are you using your lights at daytime running lights? Are you cycling them (turning them on/off) unnecessarily?

 

This happened to my 08 I was about to fix it when I totalled the car due to some old lady running a red light.

For me the whole driver side headlamp system had to be replaced. I had to constantly look at my cars reflection in other cars and buildings to verify my light was still on.

 

I've got a similar problem with my 2007 (140,000 miles) - only the right HID light don't work at all. It had quit suddenly this time. Other times you could cycle the lights and it would come back on. I figured I'd install a new bulb. Bought a Philips D4R, and installed it, turned the lights on - NOTHING. Ok, bad new bulb. Bought another - NOTHING. I checked the fuses on the fuse bank, and they all seems good.

How do you troubleshoot the rest of the system? Ballast?