but I just.. I can't lose her like this
I understand feeling like this, it's very hard to let people you care about go even if you do recognise their suffering and respect their decision, it definitely hurts a lot.
I can't say for sure, but, in this case at least, it seems as though she was acting irrationally; that method would almost certainly make her suffer more and I would've reacted the same way you did I think.
It doesn't sound like a well thought out decision on her part, and personally I wouldn't say what you did was wrong. If you'd called the cops instead, then yeah that would've been wrong to do; but what you did sounds reasonable, and what she was trying to do with that method does not.
That's just my opinion.
I intervened in a loved one's impromptu suicide attempt once, I stopped her from going until I got there at least, and that was enough time that she changed her mind about leaving then. I would've gone with her if she did choose that she still wanted it, but after she let me take care of her and started getting better after a few days, she said she wanted to try living again. When she was in less acute stress and suffering, she noticed that in the big picture, her life was seeming less hopeless than she had thought.
I was so afraid and so scared of losing her I just lost it, I dropped everything and drove several hours to be with her and look after her. My other loved ones at the time were begging me to have her involuntarily hospitalised (they didn't know her address) but I refused to physically intervene with her method of choice or forcefully stop her, that would've just been too out of line to me; I know what it's like I suppose.
I still don't know if I did the right thing by stopping her when I did, I hope I didn't just make things worse... But every time I see her smile now I wonder if the extra time and experiences she's had since recovering from that have been worth it for her... I hope they have.
or even straight up ctb with her
Double-suicides have pros and cons, you both need to make sure you've made your decision for yourself, not for the other person. Otherwise things can get messy. If you genuinely want this, think carefully about it and talk about it seriously.
I don't want to die alone, or lose a loved one to suicide when I know I'll just end up immediately following them anyway either, though.