There's a difference between "only one right way" and "you're not currently doing so hot". I mean, technically "destroy the enemy through superior harassment and recon coupled with striking when you have the advantage" is "one right way" to win basically every multiplayer RTS since Warcraft 1 all the way through whatever the latest Command and Conquer is, but that doesn't mean there's no variation in play, nor does it mean the game is a railroad because "build nothing but infantry" isn't a viable win in 95% of matches. I'm stretching the metaphor way too far now. The good news is Chris will probably post an even longer, more coherent explanation because he's an awesome dude and he gets sad when people have a bad experience with his game.
So I'll just throw out some strategic advice. First off, know that you are on a clock and at a constant disadvantage. AI Progress only goes up, except for a finite number of times when you can make it go down, unless you're playing with whacky stuff like Spire Civilian Leaders, or you're abusing the Superterminal. But I digress. Generally, AI Progress goes mostly up, and any downward movement is merely a slight to moderate mitigation of that trend. AI Progress affects what the AI will have to throw at you. The AI also reinforces assiduously, and you will basically never be able to kill faster than it can replace ships. You can only defeat it in detail, and only temporarily.
Second...yeah, that's really about it. Though I'm sure you already know, taking systems makes the AI stronger even as it makes you stronger, due to the AIP increases associated with destroying its command centers. Taking a sector can greatly increase your security, but if you don't get many goodies in that sector, it may be counterproductive in terms of resource and AIP costs. My partner and I violently disagree on this point; I'm almost always in favor of neutering/ignoring any planet that doesn't stand between me and galactic blind, she would most likely prefer to expand in a circle, first taking out every system adjacent to our command centers, then every system in 2 hops, then 3, until we win. I suspect most people fall somewhere in between our two admittedly extremeist positions. But more importantly, you may have overconcentrated the AI's attentions.
Generally when you play, as you take systems and the game starts to get serious, you will have increasingly greater planets on alert, and you'll be defending on more and more fronts, and you'll have worlds that are sacrificial and expendable, and easily rebuilt, and you'll have the planets which get serious defenses, either because they house irreplaceable resources like fabs or fact. IVs, or because they're natural bottlenecks in your burgeoning galactic empire. While this means you have to occasionally reclaim some rather generic systems, it also means the AI will be spreading its reinforcements over 10 or 15 or 20 systems instead of plopping every single ship on one of three systems immediately adjacent you.
Then there's border aggression. This is an AI behavior that I believe still exists, but might not. Basically, once the AI runs out of places to reinforce, everything in excess will just unceremoniously rush you. That would be why you're eating constant attacks in the four digit range. That, and the waves. At 800 AIP, the waves will be somewhat ugly.
On the defensive side, gravity turrets and tractors would help deal with lots and lots of ships that try to break through to your less defended systems. Placing forcefield after forcefield over the wormhole would also help, especially if your active defenses cluster around that point, or at least have it in range.
On the offensive side, you don't need to destroy every ship standing in your way, you just need to hit the targets that count. That means often times a transport full of mark 1 bombers can win a system for you. Or if not, one transport of mk 1 bombers and a handful of decoy transports. The AI hates that. Although 2k mark V's will still murder just about anything.
