So with the help of reading strategy threads here, and playing a few games of Sins, I found myself at the following point.

Small map, I'm TEC with 6 gravity wells with population centers/mines maxed out and a trade center at each one. The enemy Easy Vasari has 3 gravity wells. I have a fleet of 3 capital ships and about 40 LRM, flak, and CL frigates. I've researched lightly, enough to colonize planets, build up all of the TEC mining boosts, build some better frigates, build trade centers, basically all of the items I keep reading are the most important. The enemy attacks at a planet I'd recently captured (so it's unfortified) but my fleet is there. He wipes out my entire fleet. I have a healthy economy, so I build 4 capital ships and pump out frigates again. The next planet in the chain was previously a choke point, so it's very well fortified with as many defense structures as you may build. My new fleet of 4 capital ships and about 50+ frigates arrives and is getting formed up at the border when his fleet attacks. He has two capital ships, several heavy cruisers, I don't know how many frigates, and a ton of fighters/bombers. My fleet is completely destroyed, even with the help of the defenses.

So I'm missing something about this game and feeling frustrated enough to give up. Do I need to focus on maxing my fleet upgrades first? I wouldn't have thought one needs a 200-ship fleet to beat an easy enemy with (was 3) now 4 gravity wells. I don't know what else I did wrong though. I guess my fleet was puny? Those capital ships (3 Kols and the one that can colonize) went down fast.

And on that note, how do you ever play a large map; I'd need easily half of the fleet upgrades at my disposal to take down what an easy enemy with 4 gravity wells can do, what do you do against a hard enemy with 10 wells...you'd need thousands of ships!

So discouraged.

EDIT: I left something out. Colonizing was very slow. Planets from the start of the game had heavy cruisers for defense, and I lost a capital ship trying to take my first planet. I went back to a prior save when that happened. I don't understand how people say they colonize every few minutes; not against the fleets I was facing you don't! I don't mean enemy planets, I mean neutral planets.