Alright. Does her behavior sound suspicious? Maybe. But you know what? It sounds more to me like the behavior of a woman who has likely been exceedingly lonely for most of her marriage, and as you said so yourself, depressed, and finally has friends that she gets to spend time with.

And instead of dealing with your suspicions in a responsible, mature, communicative way, you did the following.

You accused your wife of using drugs based on her decision to expand her social life despite knowing that she has few friends and spends most of her time alone.

Then, you accused her of cheating, also based on her decision to expand her social life.

And, you had secretly purchased a drug test with the sole purpose of "catching her" in an act you accused her of with zero evidence.

Then, you sprung the drug test on her and essentially told her you were sure she wouldn't pass.

You say you don't want to lose what you have, but maybe you should, temporarily.

I don't know what you can do to fix this. You could have communicated. But you didn't.