Seeing as how you are low on happiness, it probably makes sense to destroy all but 1 City of your opponents and then (after learning Feudalism) capitulate them (stay at war with them until you get Feudalism and have sufficiently larger Power than them).
Ideally, the City of theirs that you allow to survive will be near a Happiness Resource that you do not already own, but such that their City isn't in a great enough location for them to come back into the game by spamming settlers and breaking off the Master-Vassal relationship... something like a tundra City near a Fur (Happiness) Resource would be what I'm thinking of, but other Happiness Resources could work, too.
The idea is that you will get extra Happiness for each Vassal that you control and them you can get them to give you an extra Happiness Resource on top of that, allowing your City to grow further.
As we discussed previously, you'll want to keep all of your remaining Forests, both for the extra Health and for the future potential of putting Forest Preserves there coupled with the National Park for free Specialists.
The Happiness boost from having Vassals that give you Resources won't matter very much if you build yourself the Globe Theatre but until then, you'll be desperately wanting extra Happiness... to the point that if an AI offers a Happiness Resource in trade, you should be willing to trade your only source of a Healthiness Resource just to get it... but, for example, first you'll probably want to build a Road on your Corn Resources so that they can be traded away.