I'm curious what your current interests and experiences include, and what events surrounded your recent interest in music as a career. What steps have you taken to assess your competitiveness in music with objective sources?
As aeh has suggested, life satisfaction comes from living in alignment with our values. It's important to know yours. You might find Viktor Frankl's book "Man's Search for Meaning" a helpful tool in becoming attuned to what gives you a sense of purpose. The Stoic philosophers, like Marcus Aurelius ("Meditations"), are another excellent resource to help you consider your values system.
To an extent, polymaths just have to choose a lane and commit for a time to determine fit (Newton's method for life). In your early-20s, you won't have a sense of the degree to which life is fluid, or the extent to which new paths can be forged (and with what success or difficulty). Many of us here have multiple graduate degrees - and multiple careers. It can be done, and it's common. However, it does require letting go of the idea of being "best", because anything new demands a beginner's outlook. Get honest with yourself about what you're valuing - is it internally or externally mediated? Chances are, it's both.
Suggest you keep a journal of interesting articles you encounter or thoughts that arise. You'll start seeing patterns in areas that are tangential to your math studies that do intrigue you. If music is a fit for you, it will feature prominently.
It's easy when your life has been a predetermined path to find yourself rudderless when the initial offering feels lackluster. You're unaccustomed to having the degree of agency you currently enjoy. Don't shy away from that power.
From an evolutionary perspective, humans are only just dipping a toe into the pool of actualization. The challenge of finding deep intellectual purpose - at a population level - is an artefact of the last 200 years, and even then for only some cultures. Personally, I think actualization can only ever be achieved indirectly in pursuit of our core values, not as an end in and of itself.
How can you make this process easier? Do difficult things every day. Physically challenging tasks that shut down mental chatter and require mindfulness are effective for me. Practice doing something daily that you know you will fail at. Learn to crave failure and the opportunity to do better the next time. When new skills come easily, frustration tolerance is the relatively undeveloped skill. The path you're on will require courage and grit to navigate. You may have to shed the skin you've lived in, or stich together a new one that you would never have imagined would fit. The only way to the other side is through the fire.
You can do it.
