What difficulty and galaxy set/up are you using? It sounds like you may need to up the difficulty so you can no longer coast by unchallenged on your usual strategy. The AI doesn't become fullstrength until it reaches "Tough". Above, it gets bonusses to deal with your agression. 
(Forcing the AI to use full processor power will make it behave more "natural".)
It also sounds like you can benefit from an altered set-up. If you're used to play with many manjor races and minors, try cutting back on their number, if you only play big maps, try a small one or vice versa. Everything set to abundant? Try rare. Etc. etc.
Alternatively, force yourself to try for a radically different strategy. For instance, delay building a military, give yourself different (perhaps less practical) starting abilities/a different race, or start out with enemy races set to hostile (or even at war).
Also, don't reload when you start in an unfavorable position or don't have any useful bonustile on your homeworld. The underdog position makes for the greatest victories (as well as the most punishing defeats).
I don't know if you use any tactics that the AI doesn't (sometimes regarded as "cheese"), but "gaming" the system can be a real immersion killer and give you an unfair advantage. Giving yourself restrictions on what "gaming" you can use and what gaming you can't, can really enhance your game. (I have more fun since I abandoned all-labs/all-factories-strategies, eventhough they made me more successfull at the game.)
As for the mod-question: which Galciv2game are you actually playing? Dread Lords, Dark Avatar or Twilight of the Arnor? I only mod the last one, because the ToA AI is a bit too wonky to be left to its own devices. I use Tolmekian's mod, but have also heard good stroies about MarvinKosh's and others.
If you find you've gotten in a rut, you will need to change how you play the game. I hope this has been of any use. 
