This is the one that I have been using.

Here is a link that might be useful: Greenhouse Thermostat

I just used a thermostat like you use with an gables fan in the attic. It's only been a month so I can't make any promises though. ;)

What brand and model thermostat were you using that failed so soon.
The rest of us can be sure not to buy that one.

I replaced the plastic Sunne thermostat with a metal one made by Dayton after mine started having too wide a swing in temps before it would work. My greenhouse has Dayton fans and vent shutters. My nearby Grainger wholesale supply place has everything in stock which surprised me. A quick internet search turned up the same one I have on sale as cheap as I got mine wholesale.

Here is a link that might be useful: Dayton thermostat

Dayton is good brand from what I heard. DuroStat is what my neighbor is using. Price seems right. I think specs are close. Maybe look at manual to be sure.

Here is a link that might be useful: TekSupply.com Durostat thermostat