I autocompleted the "Diplomatic Immunity" quest cuz I couldn't defeat the guards in the first room out of the kitchen area at the Thalmor embassy. Now I don't seem to be able to start the next quest in the main quest line (A Cornered Rat). Am I out of the main quest line forever or is there a way back in? If I can get back in, how do I do it?

Can you be more specific? What are you trying to do to continue the MQ and what's not happening that should be happening? Do you have a quest objective in your journal that is a followup to Diplomatic Immunity?

A Corneted Rat is a quest which sometimes just doesn't start properly. I suggest you to go back to your save before speaking with Delphine and try it again. I had this problem with A Corneted Rat with one of my characters and it leaded to situation where Esberns dialogue was bugged for the rest of game. It was annoying and prevented me from doing any of the Blades quests which were connected to Esbern.
(Anyway Diplomatic Immunity is a really fun quest and there are other ways to do it than beat the guards: sneak, sneak kill or then just RUN!)

also i heard u can wear the thalmor robes and walk past the guards lol

Really? never tried that myself. xD

If you're a high elf, and don't get too close to any of the guards.

Well that explains why they are there! I should try it next time.

Yes and no. The ones given in the other room are wrong and won't work because they aren't the hooded variant. Also, only altmer can do it if you are wearing hooded thalmor robes.

Ha.. i am must try that in next play trough.

If I'm reading this correctly you completed the quest by using the CompleteQuest console command. If so, get into the warrens, knock at the door where Esbern is locked, close the dialogue and type setstage MQ202 160. You may need to issue the command a few times if Esbern doesn't open the door. Next, type setstage MQ202 170 and talk to Esbern as usual.

Start up the quest with the console - just like you finished Diplomatic Immunity with the console...

Isn't that what you did?

A Cornered Rat is bugged and won't start with the console and completing it with console ends up in blocked Esbern dialogues for the rest of the game. I have had this problem once.