First of all, calm down again. You are freaking out for no good reason again.
Now here is the deal:
There are two sources of unhappiness:
1.) Population getting to high (overpopulation of a planet). and
2.) Too high of a tax.
There are many ways to boast happiness:
1.) Morale Buildings.
2.) Morale ability of your civilization.
3.) Special events from the United Planets
4.) Random events from planets.
To deal with unhappiness here is my suggestion:
You should not attempt to deal with the overcrowding problem. That is normal and expected. Only reduce taxes if it is above 33%.
Morale Buildings and your morale ability should be your primary weapons to combat this problem. You are Thalans, so you should bet powerful techs and buildings that gives out morale. If possible, also trade with others to gain morale techs to stack up with your own. Harvest as many morale resources as possible, and take advantage of the random events that comes along. Also, try to obtain as much morale boosting trade goods as possible.
As for the "planet ain't being used at its max potiential", it is just a stock complaint that does not have too much meaning other to give the game its character.