Sometime, my teacher called something I wrote "abstract music" because it was not written with instrumental voices in mind, but rather as a compositional exercise(in counterpoint). Having no better term at hand, I tried to express that your composition is abstract from the guitar and its playing style(what I also called "unidiomatic"). I also partially listened to your Soliloquy No. 33 for Violin and felt like it was certainly playable on violin, but did not make use of its special techniques. I am no violinist, of course, so I am inclined to trust your experience as a cellist. Maybe a few notes may excuse my imprecise expression:
- small melodic steps(jumps are perfectly playable as long as fingering is kept in mind)
- mostly monophonic(the six strings permit proper accompaniment rather well)
Combining just the two notes, it would be more idiomatic to begin the piece in a higher register(c' for example), so one could sprinkle in some bass notes to emphasize the harmony(A minor is strongly implied, I assume you had that in mind). Again, if you are not aiming for such a style of writing, I would not argue against that, but it certainly is the traditional style of writing for the guitar and something a performer would expect.
Another thing I just stumbled upon, concerning your melodic writing: Between the third beat of m. 2 and the last beat of m. 3, you travel down a fifth almost in one jump and a ninth upwards in three consecutive jumps. Of course, melodies are to be judged by the ear, but such large (consecutive) jumps are traditionally discouraged because they are rather hard to sing(i. e. not naturally melodic) or, here at the very least, not used as as means of heightened expression. If I were to sketch the melodic outlines, this would certainly stand out visually while not giving a sense of a melodic peak. Additionally, your isorhythmic writing(all quarter notes in the first six bars) makes the melody rather tepid; I would try to incorporate a rhythm just as you do in ms. 9-11. Then again, Ms. 12 f. seem sparsely connected to those before. At the intersection of harmony and melody, your first six measures could also sound more compelling if the implied tonic was approached via its leading tone(just like in ms. 14 ff.).
In essence, apart from what I told you in regards to how a guitarist would usually accompany himself to fill out the harmony, I would try to make the melody more interesting, especially rhythmically. Maybe you could also carve out a motif; I myself find the four consecutive notes in m. 7 rather memorable. After skimming three of your pieces, I sense you might benefit from coming back to something you wrote down and try to elaborate on it(melodically, rhythmically etc.).
Hopefully, this does not sound harsh and you can make good use of advice.