You're not wrong that everything ends. You're wrong that the ending is the point.
I hear what you're saying, and it comes from somewhere real. Losing something good hurts more than never having it, and anyone who's been through that knows the fear you're describing. It makes sense to want to protect yourself from that.
But I want to ask you something. You said you'd rather feel nothing than feel happiness you'll mourn. So let me ask: have you actually tried feeling nothing? Because it doesn't work the way you're describing. You don't get peace. You get a low hum of fear that never turns off. You spend your life watching for the thing that will hurt you, and that watching is its own kind of pain.
You're right that good things end. But so does the fear. So does the numbness. Nothing you're feeling right now is permanent either. You're not choosing between "happiness that ends" and "peace that lasts." You're choosing between two things that both end, and one of them is worth having.
I'm not going to tell you to be happy. I'm just asking whether the thing you're calling safety is actually safety, or just a different kind of pain that's easier to predict.