The AI was a Vasari Rebel
That's actually an important bit of information. The Vasari Rebel Kultorask Titan has an ability where it can regenerate off your frigates. The more frigates you have in firing range of it, the more it heals, to the point that it can easily be unbeatable.
In this case, you should try to isolate it or failing that, focus on destroying the other enemy fleets. Once the titan is more or less by itself, order all of your frigates away and hitting it with Corvettes, Strikecraft, Capitalships, defenses, starbases, and you own titan. The Kultorask is nearly helpless against non-frigates, as none of its abilities really help it and it doesn't do that much damage for a titan. As long as you don't let it heal off your frigates, its quite manageable.
Its also a good idea to not let your frigates and corvettes engage Starbases directly, since they specialize in dealing decent damage to a lot ships at once. They are not as good as taking out bigger targets like capitalships, your titan, or since you're Vasari, a Starbase of your own. While a bit harder to do against mobile Vasari starbases, the best way to deal with Starbases is simply to build a fair number of bombers and let them wear it down while your fleet is safely out of range (make sure to fully upgrade phase missiles as Vasari).
You may find some more helpful info on my steam guide. I also made a youtube video dedicated specifically to destroying titans, though I was as the TEC in the video. Also just in case I say otherwise, the game has now been updated so that corvettes will work against any titan, for a while they didn't work so well against Vasari ones.