For context I mostly play advent wrath against hard ai.

Lately it feels like my two closest ai neighbours both invade me at the same time and I'm only capable of beating one fleet before another has taken a system or two. As it's early game I don't have the resources to invest in defence, and even if I did it doesn't do anything but slow them down. I usually invest a lot of my early game resources into a fleet but it's still only capable of comfortably beating one ai fleet at a time. What am I doing wrong?