How do I tell my boyfriend?

It's hard for me to even admit it on here but I've been having some pretty awful thoughts lately. We just spent a great few days at Disneyland. We're doing great in our relationship. Everything's fine and great. How do I tell him that during our nice dinner tonight I was thinking about what it would feel like to kill myself? How do I tell him that on a ride yesterday I was thinking about how I would choose a knife? How can I admit to him that I was pretending to be happy these past few days? How can I tell him I've been having these thoughts when I can hardly admit to myself that these thoughts have been occupying my mind? I'm disgusted with myself and I'm afraid he'll think I'm broken. I want to be happy and great full for my life and now I feel like I'm a trader to him and myself. Should I just keep all of this to myself or should I tell him? If so, how? How can I admit that to the love of my life?