Ok here is an update:
Yesterday i restarted from one of my previous saves and tried 1 of 2 things.
1. I tried doing the missions and found out that i absoulitly hate them! Almost all the missions that AI gave me were to attack planets in diffrent star systems places where i don't know how strong the oppistion is and where they are. or something a little more resaonable attack other factions but even that was difficult to do; having the most planets under my control i controlled the most terrotory but that made it harder to defend and when ever i took a sizeable force out to attack other planets my border planets were more vulnerable. This wouldn't be that bad if a had a bunch of safe core planets... But suprise, suprise! A wormhole where hundreds upons hundreds of ships were constantly flowing out. So how do i handle doing missions and go about doing them?
The second thing i tried was to ignor missions unless they were easy for me and just build massive amounts of ships now that part is going pretty well i have split the seam of planets in 2 basically and gotten 4-5 more planets but this poses a problem too. Even with the massave amounts of ships and fleets i have constantly having to defend 5+ choke points is very hard and if i expand i risk losing what i have gained. For example: there was one corner with a 3 planet faction which i thought i could take i send a big fleet there but then lose the 2 planets they were defending and a skeleton fleet would not be enough to take the 3 other planets so leaving ships was not a option and in the end you can put millions of defendses and still get attacked what do i do?
Hmm, well, I'm not expert so, this is just IMHO..
Well, if you are already controlling the majority of space, and the AIs are consistently ganging up on you, then I would probably hunker down.
1. Do you have hanger defenses and repair platforms? Usually 4-6 hanger defenses + 3-4 repair platforms put together will strengthen your defenses to no end. At the later stages in the game, when enemies usually have bigger/more powerful ships, I usually do all bombers instead of fighters. Add some 5 or 6 light frigs + 5 or 6 LRMs, and the planet becomes a real hard nut to crack. I had one such planet with exactly this config versus a fleet with a bunch of hoshikos, a kol, bunch of frigs, LRMs, seige frigs, fighters, bombers - and my defenses were holding for a very long time with no "backup". Of course, if they send carriers with fighters, you got to micro and take out their carriers first). Point is - such a defense will do two things - repel the attackers completely (if it is not too large, eg. almost all pirate attacks) or delay things for so long that you can take other action (like bring your mobile defense fleet to assist).
2. If the enemy is attacking you with a fleet, sometimes, (if possible), the best defense is a good offense - attack their home planet. In the case of an AI, if you have a strong enough fleet and their home planet is seriously in danger the AI will withdraw (at some point) to come back and help defend. If they come back, and you are strong enough, you may even stay to finish off their fleet. They've just withdrawn from an invasion somewhere else only to drop into another big fight.. It could even be the turning point.. (But Gen. Patton did successfully do this in WWII to rescue the 506th at Bastogne tho
3. Unless you are talking about asteroids, don't try to take more than one planet at a time. There is only so much "pause, build, move" you (a mere human can do. Always favour planets over asteroids. You will eventually choke the enemy of thier supplies etc. Aside from Crystals, Metal and Credits, Planets give one very important resource often overlooked by players - logistics slots. Enemies can't easily rebuild frigate factories if they don't have enough logistics slots. If you are attacking asteroids, and you already have a good base back home, don't bother colonizing it just yet, just whack the asteroids and leave it "unowned". If the enemy needs/wants it, let them have it, but they've got to start all over. And to make thier life hell, leave a couple of frigs behind, any new construction will be very difficult, and the enemy has to commit a large force just for an asteroid? Missing the forest for the trees I think.
4. Know when to withdraw. Sometimes, harrasing the enemy is good enough. I go in with a huge fleet, whack their mining ops, frig factory and R&D labs and some of their defenses and run.. Sure, I could have stayed to fight and probably win the planet, but this causes the enemy a whole lot of trouble to rebuild their base. And it distracts their other ships by getting them to come back. While their other ships run home to momma, I jump somewhere else (easier), whack it and gain a beachhead. Build defenses like point (1) with my fleet there to help protect until the (1) defenses are stable enough.
I hope these suggestions help.. Try them on your nearest/weakest enemy. Do it one at a time.. In the scenario you describe, it is more of a war of attrition than a "crush them all" kind of battle. Again, IMHO.. Good luck!