Will I be sectioned if I tell my GP this? HELP?

Yes, you would be sectioned. It sounds a lot worse than it is. I think you'd find a hospital extremely helpful. The fact that you want to tell your GP shows that you don't really want to kill yourself - there's something left in you that's still fighting. Please tell him. If you're not going to tell him, please look into therapy. Therapists are much different and warmer than psychiatrists.