Struggling with girlfriend's death by drug overdose

My on/off gf of last 2 years died three weeks ago. She had been Fentanyl addict before we dated, but was not using at the time. While we dated, she sometimes used other things (coke, weaker opiates) and sometimes I approved of it as keeping her from anything harder, sometimes I disapproved.

A few times, she relapsed. She'd be out of contact for a couple days, later admit she had gotten some Fentanyl & relapsed. But that had not happened in six months.

The night in question, she was at her place and we video-chatted and I could see she was super ###$ up, sweating, eye-lids mostly closed, but very happy, could tell it was too intense for any of her "lesser" drugs. In fact, I'd made a point to cut my financial support so she could not afford coke or the weak opiates she sometimes used.

That night, she got got some bad news, could not cope well, needed something, scored Fentanyl from a friend. I realized she must be on it, though I don't know what it looks like to be on it, just knew she looked way too high, so accused her of being on Fentanyl, she admitted it... Then we talked for another hour or so. It was a good talk, sweet.

I never thought she was OD-ing. I guess I though she'd had so much past experience with it, she knew her limits and it did not even occur to me to worry about that. But after we got off phone, I went to bed, and later she passed away. Maybe she took some more after we got off the phone, or maybe she was already OD-ing and I just did not recognize the signs.

I had thought of going over there that night, not thinking to save her from possible OD, but from doing something stupid while high like trying to go for walk at 3am or drive somewhere... But I thought she might be too high to buzz me in when I got to her apartment (30 min away).

Now I beat myself up. She was so sweaty and her eyelids so droopy, so too-relaxed... why didn't I think it might be too much? Why didn't I think she might need help? Why didn't I go over there that night?

The whole relationship I felt like a knight trying to rescue a damsel in distress... And I failed as badly as anyone can fail. I realize even cutting her off from resources to get lesser / weaker drugs to some extent forced her to get the only salve she had access to for free (rich friend who was Fentanyl addict and would share with her if she asked). If I'd let her do some norcos, some coke, gave her money for more xanax or weed, maybe she would not have used Fentanyl... Trying to wean her from drugs, I drove her to the worst one that killed her.