Real Racing 3 won't let me sign in to my account ("Guest" problem).

This was recently fixed.  Open RR3, go to Settings gear, click on next Settings gear, Download All Assets, and you should be signed in again.  

i did it, but guess what? when i restore the data i have now only 7 cars instead of the 80+ i had, and 99% of my progress IS GONE!!!!!!!  wtf!!?!?!?!!?!? 

I don't know what to tell you.  I didn't need to do a cloud restore as all my M$, R$, Gold, cars, etc. were still there after I clicked the Download All Assets button.  I do manual cloud saves daily so it wouldn't have made much of a difference for me if I did use Cloud Restore.  When the problem first started I did lose all assets but luckily cloud restore did still work and I was able to recover everything.  Another time while trying to fix the problem I lost most, but not all of my progress then I noticed RR3 had used an old google account instead of my current one.  Maybe that's what happened to you.

im using the same account. and still, when i restore from could, it restores like 1% of my progress and 99% of hard earned progress is gone like its never been there!! and EA wont help.. all they do is letting me chat with some half human chatbot that gives no solutions at all.

I know that really sucks.  When you were playing as Guest did you still have 100% of your progress?  If so were you regularly doing Cloud backups manually?  

ME7 has a good video (which I used when filing my ticket).  ME7's Video

https://www.youtube.com/watch?v=_15i959mGSw

It took several days to get a reply but eventually someone should get in touch with you in-game.  Keep checking the Support section and when they have responded to you there will be 1 in a red circle on the Support button.

i did it few times already. guess what? nothing! still 99% of my progress is lost!