backpacker in ESTA VWP for 75 days a problem in crossing the border?

I want to go to the USA UU For 2.5 months and I am a little afraid because I will have difficulties to enter because it is for a long time. I'm going to the backpack and I'll stay with my friends. I also have two flights inland, I will spend two weeks in 4 different cities and then I want to go on a trip, maybe rent a car. I'm a student but I do not have a job because of that. Has anyone traveled there for so long and experienced some problem? I am a woman, white and from Germany. I also do not know how much money I'll have to try. any experience would be appreciated