Hi!
NeuralTech, GalCiv-2 is a 4x game. Those 4x stand for eXplore, eXpand, eXploit, eXterminate. If you want to be a successfull 4x player, you simply must follow the path of the four X:
1) Explore: find what's in your space and beyond, to plan your development properly.
2) Expand: take what you found as fast as you can. The goal here is to simply GRAB EVERYTHING. If you don't, others will. The one who grabs the more will in the long run win. The catch in 4x games is this phase shall NEVER end. You must grow and expand until you win.
3) Exploit: develop what you get, use whatever means you have available to make you stronger, or opposition weaker. Know thy enemy.

Only so you'll be able to exploit their weak spots. And when you start exploiting them, you'll do so after thorrough preparation of several months: proper fleets in proper positions, your enemy diplomaticaly weakened if not at war you've paid and financially broken...
4) Exterminate: do that FAST! Do that to the one you can subdue the most easily. Use what he has left, proceed with the next weakest. Repeat until you win.
Looking at your post I'd say you failed in the second phase: expansion. Starting in a dead corner doesn't help with the expansion, you know.

Starting next to the opposition also not, especially if they want the same things as you do. And failing to build colonizers FAST to take all planets close to you wil not make you bigger. So in the next game you need to:
- turn off blind exploration,
- play a race with stellar cartogaphy,
- use CTRL-N if you start in a tight spot,
- produce a colonizer on your HW each 3rd turn, until there are planets you can settle, or you're losing too much money each turn, that "too much" being proportional to your economy. If you can't cover most of it by rising taxes to general approval about 50%, it is to much.
To quote CraigHB: "If the economy is tough in Dread Lords, it's brutal in Dark Avatar." I'm sorry you found that the hard way, but that's the part of learning experience. Here you might find useful my post
Playing with lots of planets - how to financially survive first year? usefull for further advice.
BR, Iztok