If you feel overwhelmed, try turning down your galactic game speed.

^^ As above play on slower speed setting and pause always unless you have to unpause briefly to move stuff !!

On expert and in general if struggling:

Building strong space defenses or anything military at all really except for republic advance dockyards at Kuat semi-early is usally not very efficient at start since they fall easily and are expensive to rebuild. And you need every credit put into research and to build up your economy at start. Space defenses can be seen as support i.e. also need a strong fleet close e.g. kashyyk, Lantillies etc. to be of any use really and you just don't have that at start.

I usally at the beginning focus instead on holding ground on chokepoint worlds, it is much easier and usually have zero losses even if they attack with 50+ units. Just preferably have one arty, 2-4 AT-PT and something to kill armor (e.g. 3-5 UT-AT, AT-TE, TX130 (rocket)). Rush the big build pads, build repair stations, and defend around those. Easy peasy.

If you get constant invasions you probably need to revise your strategy to become stronger more quickly. High income will enable that. You need to be offensive from the start and aim to have say 20k income or so by turn 30. Mines, mines, mines! Some freighters. Some buildings in space and on ground that provide +75% income are Ok also on VERY high base income worlds early on. Remember they only give +75% to the BASE income so mines are generally better.

"nervos belli, pecuniam infinitam"

You have to pause the game anytime you aren't moving something. Especially in early game. There should be no downtime where the game is left un-paused, because the AI acts as if they also have the ability to "pause the game" where they can do 15 different tasks at a time. Meaning they can be building 5 fleets at the same time, while you leave the game un-paused.