What civ are you?
If I were you I would:
1. turn science to zero
2. disband unused obsolete units
3. rush marketplaces with the increased taxes (don't build them in totally corrupted cities though)
4. raise science rate back up.
5. are you religious? after you have markets, go to republic
Other points to ponder:
Are there luxuries nearby or unclaimed on an island?
Start a war with a country and seize a city on a luxury. abandon or raze it and plant your own city there.
1-2 techs behind isn't so bad, I've been in much, much worse. course, it depends on the techs
you have knights? Do they have muskets? if no gunpowder, then attack using knights. are you closer to cavs? wait till those. Brute force can usually defeat smaller numbers of better defenders, if you attack en masse. Try to take and hold a spot and either hole up there or attack other cities. get a friend to attack from the other direction and distract.
*don't attack the french if they have musketeers, will trigger their golden age probably and they will be much stronger.
can you not trade for any luxuries?
I usually try to find luxuries on my continent early, then send a settler over to it ASAP, to claim it for later. (and hope its not attacked)
Here's some help, I'm sure there's better advice out there or in the strat forum.
I would advise against selling temples. if you are already having happiness problems, this will only make it worse, though on regent, there is more of a tolerance of citizens.