How to Target Chord Tones in a Solo
Playing 'in key' is only the first level of improvising. The next step is noticing that every chord inside a progression has its own 'home' notes, and aiming for them deliberately instead of assuming any scale note sounds equally good over any chord.
What chord tones actually are
Every chord is built from specific notes - a C major chord, for instance, is C, E, and G. Those same notes are almost always part of the scale you're soloing over too, but they're the ones that sound the most stable and 'right' exactly when that chord is sounding in the accompaniment - the rest of the scale's notes create a light tension that asks to be resolved.
A simple drill: target notes on the strong beat
Take a two- or three-chord progression (see the chord trainer) and, on every chord change, deliberately land on one of its notes right on the strong beat of the bar - the rest of the time you can play any scale note freely. This is the fastest way to hear the difference between 'just in key' and 'landing on the chord'.
Why this changes the whole feel of a solo
A solo that follows the chord changes by landing on their notes sounds like it 'knows' what's happening in the accompaniment - listeners feel this even without understanding any theory. A solo that just runs the same scale regardless of the chord changes sounds correct but flat, as if it were laid over the accompaniment rather than connected to it.
You don't need every chord tone memorized upfront
A beginner only needs to remember one note per chord - usually the root (C for a C major chord, for instance) - and aim for it on the change. The rest of the chord tones get added gradually, as your ear starts telling them apart, rather than through memorizing theory ahead of time.
How to train your ear separately from your hands
Listen to a progression without the guitar and try to guess ahead of time which note will sound 'right' over the next chord, before checking it on the neck - this trains the actual auditory link between a chord and a note, not just finger muscle memory, which works the same regardless of what's actually sounding in the accompaniment.
Try it in practice
Open chord trainer