You just need better tactics. The AI will most always have better technology than you do until you earn it.
- Go out and explore the galaxy. Find strategic objectives that give you ships, toys, and resources. Note that this also allows you to get more knowledge to get better stuff.
- Don't go around taking every planet because you will quickly lose the arms race (or have another birthday in the process of grinding out a win).
- Take out guard posts, but leave one warp gate next to a well-defended planet. This will allow any incoming waves to go where you want them to and prevent buildup.
- Poke the bear every now and then to decrease build up, leading them back to your well-defended planet where you will destroy them.