Hi, I heared that when i7-3770K temperature is 65 celsious and up, he losing performance, is that true? Should I worry about it, im feeling very sad and bad for this. How can I be sure that my cpu is not damaged?
You have nothing to worry about. Those chips aren't in danger until they hit the 90s and chips nowadays will shut off or throttle when they hit their limit. 65C is perfectly normal under load.
stock cooler is terrible man, you should get even a cheap aftermarket cooler, and try and keep temperatures below like 65C, its technically still safe at 85C but for games id expect under 65C imo.
You don't need to buy an H100i man, that is overkill. If you are really concerned just grab a Cooler Master Hyper EVO 212, it is less than a 1/3 of the price of that Corsair and will be more than enough for your usage. Your CPU did not take any damage, it will shut off before it really takes damage.