Shivans have numbers on their side. They can sacrifice a whole juggernaut fleet as if they were just zerglings to get the job done.
The GTVA has the advantage of adaptability. If they capture Shivan technology in its entirety, they can not only replicate it exactly, but also make significant improvements on it (SF Mara, Sekhmet, UD-8 Kayser), thereby closing the technological age gap almost instantly.
In addition, the GTVA fighter wings, one-for-one, are better than the Shivans, since they need to be in order to stand a fighting chance (GTVA has limited numbers, so they need to preserve as many pilots and combatcraft as possible to beat off Shivan swarms, hence the rearm/repair ships + generally higher hull hps). This also means, many of these pilots gain lots of experience in combatting Shivans, which = better survival + kill more Shivans, while the average Shivan pilot is basically a rookie, and dies a rookie.
GTVA also has better anti-fighter defenses (Leviathan, Aeolus, Deimos and Sobeks all eat fighters + bombers for lunch).
Another point is that the GTVA has stealth fighters, which you could send as surgical strikers to take down key, weak targets (freighters/transports, cargo, subsystems and turrets on Shivan warships...etc.).
The most important advantage is that, the GTVA has survived two major Shivan wars already. They will be able to survive others.
