Been playing a long time. Still trying to figure some things out
...
How do you select the right note (this is my Achilles heel)
...
I don't know where you are in terms of genre or style, what your goals are or what you think your difficulties are so it's difficult to pitch a response, but here are some general principles that might help...
Notes don't tend to exist in isolation. Rather they exist in small groups or clusters. Groups of notes that appear together at the same time are called chords, and those that appear one after the other over time are melody. Generally, the notes (pitches) of the chords and those of the melody are taken from the same larger group, known as the key. Keys consist of one or more scales, which are just subsets of all possible notes.
So your first task in note selection is knowing which notes are available to you...
The most important note in a chord is the root note, and most chords in most music appear in root position, which means the root is at the bottom. So, as the 'keeper of the bottom end', the bass players' focus must be roots. That means being mindful of them even if we're not actually playing them.
The second most important note of a chord is the 3rd. This usually defines the 'quality' of the chord as major or minor. The 'quality' of the chords within a key follows set patterns. Look up 'harmonising the major scale' as a starting point for which notes to focus on when building bass lines.
Harmonising the minor key is a bit more complicated, but the process is the same.
Bear in mind that putting notes other that the root at the bottom of a chord is a powerful tool that can change entire harmonic structures, and not always for the best! Take a C major chord and put E in the bass, it's still a C chord. Put an A in the bass, without changing anything else, and suddenly you now have Am7!
In terms of melody, individual notes are like letters in a word - meaningless when looked at alone. Melody is made by small groups of notes arranged in rhythm over time. Within these smaller groups or 'phrases', melodies tend to move by step, that is to say notes next to each other in the scale, with larger jumps being made between phrases. Again these notes are usually taken from the key, and usually revolve around the notes of the chord being played at any given time*. So if you are looking to write a melody (and bass lines are often melodies in their own right) or improvise a solo the notes of the key, the subset of notes used in the current chord
and the notes of the next chord are the best places to look for clues of what the right note(s) might be.
* Music is frequently composed the other way around - taking a melody and adding the chords (harmonising) around it - so this 'backwards compatibility' should come as no surprise.
YMMV