First, let me affirm that you do have accomplishments--and I refer not so much to the typical career and academic markers, but that you have managed to maintain some level of optimism that this (your existing degree, career, earnings, etc.) is not all there is. The psychic restlessness that you describe isn't entirely a bad thing, even if it may cause you some distress at times, as it has also inspired you to continue growing, learning, and exploring new fields and interests, seeking something more than external markers of success.

Second, I would encourage you not to view pursuits such as music as something in which you are two decades behind. (And I'll note that there are some areas where intensive formal training is not even recommended by all teachers until entering adulthood--for example, many vocal coaches of the bel canto school will not begin formal instruction of male vocalists until they are 18.) Instead of regretting the years when you were not training in a particular area, take some gratification in growth and forward progress in the present, and in the delight of the study itself. Regardless of ability, everyone has the same 24 hours a day, which necessarily limits the maximal development of every potential in a multipotentiality individual. This is not personal failure, just a practical manifestation of being finite creatures restricted in time and space.

Not to mention, the assumption that you cannot "catch up" to those who started in childhood is not necessarily accurate. Amelita Galli-Curci, the Spanish-Italian coloratura, was a conservatory trained pianist, but a self-taught singer until she was well established in her concert career. The Russian composer Alexander Borodin was a professor of chemistry and medicine, but a self-taught composer who began studies with a musician only in his late 20s.

And with regard to "amounting to anything," this is more metaphysical than otherwise. What determines your value? You have already established that it is not your financial earnings or your academic credentials. Respectfully (without knowing your faith, political or philosophical leanings), I would encourage you to reflect on your own values, and to observe those around you who appear to have a sense of purpose and life satisfaction not based on shallow accomplishments. They may or may not be your intellectual peers, but if their experience of success, satisfaction and community arises from something other than accomplishments, there may still be gleanings for you.
...pronounced like the long vowel and first letter of the alphabet...