I think you may have misunderstood the notion of fixed vs growth mindset.
Having a fixed mindset doesn’t mean you don’t believe you can accomplish any goal. A fixed mindset means you don’t believe you can always learn and improve, no matter what. There’s a big difference.
I am 39 years old and a terrible football player. If I believe I could never represent England in the football World Cup that’s not a fixed mindset. That’s realism.
The key to a growth mindset is knowing that despite whatever my current ability level might be, I can always learn and improve.
I may or may not achieve certain goals. The point is it’s about learning and growth, not about the goal.