As the Advent, I can have a titan with 6 caps (4 being shield re-generators), 10 shield sharing cruisers and a swath of 20+ frigates and it still seems like I can't put down an enemy fleet half that size.
Your fleet is all defense oriented. You don't have enough offense (killing power). You need more than "20+ frigate", and less than "10 sheild sharing cruisers". Progenitor Caps with Shield Restore are great defense, but the Malice ability can also be used for great offense. And don't get 6 caps, until you've researched tier 8 Mass Transendance as GoaFan suggests.
Which Advent faction are you playing? The Advent Rebel titan's Chastic Burst is an awesome offensive tool.
What level of AI are you playing? Hard and above get a significant cheat. In Rebellion Hard now gets 1.7X our income/metal/crystal. Unfair 3X. Cruel 5X. Viscious 7X. And to my suprise, Normal now also cheats slightly at 1.1X. (Last time I checked, back in Diplomacy it was 1.0, 1.5X, 2X, 4X, 6X).
Yeah, like Turchany says, Starbases work well against the AI. Especially with Meteor Storm, and maybe Mass Disorientation if you are fighting against a titan.
You may also be investing too much into econ, and not enough in military early on. Three Civs is ok against the AI, but that would never work online against human players. Remember it takes time for the investments in econ/civs to payback, often more than 20 minutes. So be efficient.