Wow, why is it so hard for you to say what the problem is? It’s going to be a bit hard to get any advice.
I assume you wanted children but are sterile? You can still adopt. Even if that’s not what you mean, there are always alternatives in some way that let you achieve what you want, maybe not exactly like you imagined it, but a similar thing.
Everyone else already has said the “general” stuff that can be said without getting any more information from you. If you really want help, you should be ready to share some more information and answer questions people have, as well as answer people’s comments with sharing your true thoughts, not holding back like you did in your main post.
In the end, how do you get over this? You understand rationally that you can never plan the future 100%. You understand that once people achieve a goal/plan, they don’t become happy, they just try to achieve a new goal/plan. To become happy, you have to be happy with what you currently have, no matter what that is.
You don’t have to have an identity to live. Identity is irrelevant. It’s a thought-up human concept that in the grand scheme of things has no meaning. What does it matter what your identity was 100 years from now? 1000 years from now? A million years from now?
And once you rationally understand these things, you have to convince your subconscious of them as well, which is much harder and requires lots of practice.