If you are dead, you can't make more babies, and if you cant make babies, your species will die off. It's an oversimplification, but hopefully you get the idea. Millions of years of evolution has ensured that animals survive long enough to make more babies. There isn't an off switch for this, it doesnt go away as we get older. So it stays hard wired in our brains. You're afraid, because evolution made you afraid. Theres nothing wrong with you for this, it's now just a hurdle you must overcome. Wishing you the strength to do do what you feel must be done!