I have one city, its not small and I know that overbuilding a city causes instability. I see how at least half my districts bring -10 to stability. I'm having a hard time understanding how to fix stability issues. I can only build one watchtower right now,  I have one fort. Is there a good guide to stability. I'm sure I'm not seeing something.

Depends what era you’re in. There is a research that doubles garrison stability so it’s plus 10 per garrison. I usually build a couple of those if my stability is in danger. Also wonders and holy sites are great boost as well. Ofc do these all after you’ve built the stability boost buildings like the well and aqueduct. 

In the earlier eras, you have to balance constructing garrisons with constructing districts and attaching territories. In later eras, however, the stability issue is more or less resolved by commons quarters, though they do need to compete with other districts. I would recommend you don't invest in a lot of civics, and judiciously buy luxuries in the early games, which has an added benefit of diplomacy.