Heeeyyy! Oh yeah, worst nightmare-not. This is where you need some patience and skill. First slow done man, way down-don't need the for sale sign.
First back off the bolt; quit trying. Spray some PB Blaster on it(no not WD 40). Next take a chill. Come back in 10 mins. If you rounded out the bolt a fad you don't have a handy dandy stripped bolt socket grabber socket, no sweat. Have a file handy? If so great. If not get one to reshape the bolt to a new size.
Here's the deal. It's highly recommended that you pull the top two bolts on the tranny to avoid this very same problem; then pull the rest. Sounds like the weight of Tranny is laying heavy on the bolt.
squirt the PBB on both ends to ensure penetration. Next, Square off the busted ends of the bolt head. Fit a smaller socket on it. Use a hammer to tap it to fit. Tilt the tranny-adjust jack accordingly, and attempt to loosen as you go. Problem is not a seized bolt-it's rare but a binding bolt due to wrong order of removal. Dropped mine about 15 times now. Take your time. When you do get it loose. Leave the left side bolt just below the mating long bolt. Replace with a new one. It's less than 5 bucks at the dealer but they may want you to buy a set of 4 runs pretty cheap.
Remember, reface the bolt, then tilt the tranny to reduce jam on the bolt. Should be a breeze. Good luck.