I started playing and learning back in 1963, and aspired to be a fingerpicker right from the git-go.

At first, of course, I simply played arpeggios, and then progressed to using the same little "pattern" for each measure of a song. The pattern that I found (in a Stephan Grossman book) featured a steady alternating bass to be played by the thumb, along with a series of notes to be picked by the finger(s), including a "pinch" (simultaneous bass and treble note) to begin each measure, followed by a series of treble notes played in-between the bass notes.

The next step was to insert some simple walking-bass sequences at the chord changes while maintaining the repetitive pattern ~ and then the next BIG step was to learn entire fingerpicked pieces, which I approached as sequences of patterns, where each measure was played to its own unique "pattern." For example, pieces like "Freight Train," "My Creole Belle," etc.

I had always read about how traditional Travis-type picking and Delta-blues picking, etc., featured a treble melody-line played with the fingers "independently" of the steady on-the-beat bass line played by the thumb. I found it impossible to make any progress, or even to get started, when trying to think and to play an "independent" melody while keeping up a steady, correctly-rhythmic bass part. I was only able to play labriously learned measures of fngerpicking, eventually falling into an easy steady rhythm. I had to learn semi-complex pieces by rote, and then to play them over and over again, for years, before my fingers were able to develop their independence from my thumb.

Only by actually playing musical phrases, and entire songs, which combined a steady bass line beneath a more complex melody, was I able to develop a muscle-memory of the independent operation of my left thumb vs. my left index-and-middle fingers.

I think it was a good 25-30 years or more-or-less "rote" playing of pieces learned laboriously from tablature before my thumb and fingers finally became sufficiently independent of each other for me to play freely on the upper strings while keeping a steady thump-thump going on the bass strings. It shouldn't take that long for someone who keeps at it ~ I gradually began playing less and less for several years, eventually almost stopping completely, before getting back into playing much more recently.

What I'm trying to say, in summary, is to be patient with yourself and keep at it. Ingraining the many aspects of playing an instrrument into your brain and into your muscle memory is a very complex process that requires repetition on a regular basis. Don't be disappointed with yourself, just keep at it.