I feel your pain. Seriously, pick up some DLCs. They help.
Deploying a belt of picket ships with vast sensor power around your periphery tremendously benefits. Always get systems of 2 or more planets fully crunk with 4 economy star bases (ensure to upgrade as new research, manufacturing and economic modules come online).
Also, are you deploying military star bases near the enemy's lane of advance? That and adding additional commanders (usually I have 2 commanders and 1 admiral for a hard pressed task force). There are mercenaries available that diminish an enemy's attack/defense values and boost your attack/defense values.
Use frontline shipyards sponsored by core worlds for faster replacement/reinforcements and chase down logistics to create larger fleets
Get as many leaders as it takes and max out fleet hit points and star ship construction in the Govern field.
Set the auto save to two turns. When you suffer a disastrous battle like Lepanto, Midway, Wolf 359 etc, "jump" back in time to better prepare for the oncoming battle.
luceo non uro