Achieving the correct pressure on the strings with your left hand is a very common problem for guitarists and one that is often overlooked.
As you said you have been playing for quite a few years with reasonable facility, and when that is the case we don't tend to look to our technique when we want to improve.
In developing my playing over the last year I have been thinking a lot about the left hand and it's importance in fluency and musicality.
The main problem that we have is that the left hand can easily become 'blocky' as we learn a piece, with the main influence being on shapes, finger patterns and the easiest way to finger notes without enough reference to how we want the music to flow.
This inevitably leads to too much pressure and fingering or holding notes unnecessarily. I am just learning to transfer weight from finger to finger as I play to help get a real sense of direction and to balance my hand.
Firstly you need to learn what the correct pressure feels like; place your middle finger on an A on string 3, fret 2; now relax your finger so that it is only resting on top of the string resulting in a dull thump when you play. Now play continuously with your right hand while gradually increasing the pressure with your left hand. As soon as you hear a clear note stop and make a mental note of what your left hand feels like - you will be surprised how light the touch is! Try this with different fingers on different notes around the fingerboard, and then with simple chord shapes.
The feeling changes depending on the difficulty of the shape or stretch.
You can then move on to playing simple 2 octave scales, no open strings, using thumpy notes throughout ie no pressure (no note is sounded just a thump generated by the right hand playing and the left preventing the string moving). It is great when you feel the freeing up of your left hand and you start to be able to move around more easily.
I also practice pieces with this Thumpy method although that does take more discipline because you will be able to hear the open strings as clear notes. You have to keep the focus on the feel of the pressure under you fingers rather than the sound you are making.
To improve the balance of you left hand and the transfer of weight from finger to finger it is best to use simple walking exercises.
Choose a comfortable position, 5th for example and start off playing patterns on string 3 - 1234, 4321, 2314, 1324 etc.
Walk from finger to finger and make sure that you are balanced and relaxed on each one. Then move on to doing these same patterns across the strings.
Start on string one and as you move to string 2 make sure that you take the weight and balance of your hand with you - by this I mean do not just reach across and feel for the note but move positively onto you finger so that you could maintain that position all day without getting tired (like standing on one leg!)
I have also discovered in explaining pressure to a pupil that you can slide a piece of paper between the string and the fingerboard while you are playing a perfectly clear note!
I hope that this is of some help. Doing this work has helped me to play more fluently, quickly and musically - the latter being the most important!