We're all different, but I think maybe committing makes all the difference? When I hung myself it was a really shitty attempt, since I was in jail and had to make do with a rope made from a bedsheet. But I was 100% committed, I thought I'd break my neck by making sure the drop length was 6 feet but I'd still be fully suspended. I didn't know the sheet would stretch like it did and only achieved a partial (my foot hit the ground and took most of the weight away from my neck) but I still lost consciousness incredibly fast, ~10 seconds from what I can remember. There was only the briefest moment of panic/pain before I forced myself to stop struggling and I was out, bright light and blissful peace and all that stuff you read about.
That said, @Anonymoussn is right - if you haven't thought out/planned out and committed to every aspect of your plan, its just an impulse attempt that's going to be more likely to end in damage and forced treatment than death. That's not me getting on your case, that's me trying to help you make sure your situation won't just be worse after tonight. I wish you peace and happiness, whatever happens. Much love.