It was hard not to adopt a defeatist attitude.

It was hard to not adopt a defeatist attitude.

I confuse myself all the time with the usage of to in circumstances like this. Help!

I really like the first choice "It was hard not to adopt a defeatist attitude" because it sounds and reads better to me.

I'm not sure of what's "proper," and I believe both examples are correct. (?)

The reason I prefer it:
"adopt" is a verb, so those two words in that exact order "to adopt" fit together so well!

I remember learning Conjugating Verbs, where we stated those two words first:
"to adopt"
Then we went through:
"I adopt; he adopts; they adopt; we adopted; she will be adopting" etc.