FL Studio help

Sorry cannot help there I only know Ableton and Logic…

I play the melody line that I have in my head on a MIDI keyboard into the software and when I hear that one of the lines I had in there did not match (e.g. bassline) I create a new one.

But this sounds to me like a basic problem not the software. My best guess would be you cannot hear key notes and do not know which notes harmonically play together.

Learn basic music theory from either books or take lessons. Picking up an instrument is also always a good thing. Keyboard is mostly used by producers so to learn to play keyboard is good, but through real lessons no shortcut guides, as you need the music theory behind it.

Well you only learned to play notes on an instrument, not the theory behind it is my guess.

I can recommend the study of the following books (in that order and yes ALL of them not select one):
Total Harmony by Stefan Kostka
Theory of Harmony by Arnold Schoenberg
Twentieth-Century Harmony by Vincent Persichetti
What to listen for in music by Aaron Copland
How music works by John Powell
Infinite Music by Adam Harper

Of course if you can get lessons with a good teacher, then it would be even better. However it is not easy to find a good classical teacher that can also think about electronic music.