Random, unordered thoughts on your game.
You've got a two front war with people who don't want to acknowledge your envoy. Not good. You've got a Military Alliance with Portugal vs. England for another 14 turns, which means if you make peace with England before the 14 turns are done, you break your agreement with Portugal, and that can have effects the rest of the game.
Of the two foes, England looks to be the more dangerours. Germany's stack is rather small and can be whittled down before it can do any damage. England is already doing damage. Southerner Rohan looks lost but Elephantium may be able to save it. England may have destroyed Russia since Moscow and St. Petersburg are both English.
I would try to make peace with Germany first and expect to slug things out with England until the MA expires and then make peace. And get things straightened out.
You are going for a Conquest win yet your government is Republic. Monarchy might be a better option since it has no War Weariness. Or maybe not. You have four luxuries connected and that will help a lot, once you get markets in your core cities.
Five workers and eight slaves is not near enough support staff. A good rule of thumb is at least one worker per city. At least one per city; two is even better. That would allow you to have worker stackers that could move into a tile and do something (mine/road) in less time than a single worker. Three workers working solo will road three grass tiles faster than a stack of three workers attempting to do the same. But if you need to road directly from The Great Fields to Pelennor Fields, a distance of three tiles, having stacked workers is the way to go.
Others have spoken about your military and you yourself know that something went wrong, or you wouldn't have posted a save and asked for help. Which is the smart thing to do. You do need barracks and you need them early. Barracks produce veteran units, which are sturdier in combat (the AI uses a lot of regulars units) and promote to Elite in reasonable numbers. The first few units you build won't be veteran but as soon as you get that thing built, only build vets when you build units.
General George S. Patton said:
No bastard ever won a war by dying for his country. He won it by making the other poor dumb bastard die for his country.
That applies in Civ3 too.
Veteran units are more likely to survive combat and can heal and be used again, which makes them more effective. They get to kill more units. Units that don't survive combat means that they have to replaced and get built from scratch and then moved to the front lines, all which takes time. Wounded units can get back into action much faster than training up the newly commissioned 31st Sword Divison.
Also, the way the AI rates the military is based on Attack factors/Attack points and not on Defense. Thus, if your military is all Archers (2.1.1 units) you will appear to the AI to be stronger than an all Spears military (1.2.1). This factors into how the military advisors ranks our nations armies compared to yours.
I do think you are in a tough spot. I do not think the game is lost.
This thread might be helpful, too:
Recovering from Last Place after Expansion Phase, Step by Step.