Hello. I'm so sorry for what you've been through. Feel free to share if you'd like(if it helps you or you need to talk)
12 mm isn't always super strong for full suspension. Generally people use 15 to 20 mm for full. 12 mm would hurt more because it's a smaller diameter which means that all of that force is pushing in a small area on your neck, which will hurt worse. Plus it may not be strong enough to hold. You'd have to know the strength of the rope and all. It could also snap immediately if it's weak, being that you jump.
Hopefully you've tried all your other options and this is just the last resort. Suicide is a right, definitely. But I just hope you're not rushing into it too much. Obviously you're serious because you are using this website. I just hope it's not something you decided to do within a couple of days since something happened.
If you attempt, just be sure not to be found early so you don't suffer brain damage. This is a high risk if you're found. If not, all should go well if the rope doesn't snap.
Remember that there's no shame in backing out. There's always tomorrow. So if you feel doubts, why not hold off? We're here for you either way. No judgement.
I truly, truly hope you find peace and happiness. You don't deserve to suffer and I could only hope your pain comes to an end.
Best wishes.