I'll tell you the secret: virtually every GalCiv game can be won, if you don't play the way the game was meant to play; if you don't play by your strenght, but by AIs' weaknesses.
AIs in game can't addapt to certain strategies, can't change their responses, and fall to the same trick again and again. In my case, it was interesting to try and to test these strategies and weaknesses, but at the end it feelt like playing the same game again and again.
In your case and with tough AI IMO you don't need to play that way. Just try to expand more agresively at the beginneing of the game, and use diplomatic options (that 2000 years old roman method of "divide and conquer") more often in mid game, and you should be fine.
One more tip: going against the odds, using a tiny window of opportunity, and prevailing, usualy made my most memorable games. Before you quit, try some options. You can't lose more, can you?