Are you seriously contemplating whether you are mentally ill or not?Having Neptune square Mercury and Saturn trine it does not (automatically) result in mental illness. They do not cancel each other out either, you will feel both influences, depending on their orb of course, but Saturn will help ground your mind and organise your thoughts, while Neptune will scatter them which can make you confused.
Mental illness usually shows up as a combination of patterns. From my brief observations these show up as a combination of aspects, usually ones that make your mind and/or your physical energy irritable, ungrounded, frustrated, angry, explosive, etc.
So, for example, Mercury and Jupiter (lower and higher mind, right/left brain) in opposition, both squared by Neptune. Then add to that some volatile aspects such as Mars, Pluto and Uranus all in difficult aspects to each other... Uranus in a difficult aspect to Mercury alone can also 'feel' like it as your mind races endlessly and can overthink far too much.
However, it should be noted that some of these aspects can also create brilliant minds, even if the person appears crazy to others they may also be very smart; but again you'll have to look at everything to even consider 'illness' as such.