I'm afraid to tell my fiance about the broken collar bone out of fear it might life in a negative way. Am I right?

Common fracture. The collar bone is one of the bones most often broken. It sits in a sausage like sleave and often simply cracks without separating or separates a bit. Rx is commonly just keeping the shoulders back & maintaining allignment. Rarely (in major trauma) it can puncture the lung on that side. Once healed, life goes on. I see no reason to think it should hurt you or limit your life.
Don't be afraid. I would tell the truth here, you might be surprised at how understanding the response might be, and will be hard to hide this and will be more difficult to explain later the fracture and why you didn't mention it before.