Just don't forget to check whether you actually need (or will soon need) the happiness bonus. Don't need it? Don't build it!
Okay, I guess this didn't get my point across very well.
I certainly didn't mean "Don't build forges unless you need the happiness". Rather, I meant "Don't build forges
for the happiness bonus unless you need, or will soon need, the extra happiness". To which I might have added "And make sure you've actually got/can get access to the requisite resource(s) if you build the forge for happiness reasons".
Of course, you can never have too much happiness. But you can prioritise happiness-giving buildings when other builds would be more useful.
To clarify my general anti-buildoholism remedy:
Buildings are good. In fact, all buildings can be useful on certain occasions (yes, even castles). Some buildings are helpful in almost every situation, others in some situations, and some are rarely helpful at all. But...
It is not necessary (or sensible) to build every building in every city. Indeed, it's not always necessary to build any particular building in every city. Every building has its uses, but, equally, every building has its cost (that being primarily the opportunity cost of choosing that build over the other available choices for your city).
So, the key to overcoming builditis, imo, is to recognise the opportunity costs of a building, to consider whether you actually
need the benefit(s) you will gain, and to ask yourself whether some other option could prove more useful to your chosen strategy.
(This is undoubtedly complicated by the multiple benefits given by many buildings. But that does not diminish the value of the maxim "Don't need it? Don't build it." It just makes it harder to judge which building is most useful in a given situation.)