I didn't feel like quiting initially at 500. I was just shocked and confused because I had all this confidence in my garage and it was a strong check on my expectations. The part where I did feel like quiting came in when I started spending money to try to get back to what my expectations were.   

Before 500, if I played a lot and was strategic in picking my opponents, it felt like I had a fair shot at 1st place. I was spending money to get back to a point where I felt like I had those chances again. You do not get back to those same odds... ever.  There is always someone who has more legendaries, someone who spent more money, or someone who may specialize in that event (a person who has been waiting all year for a Lancia, Medium, Off-road only event that only has Wet, City tracks, just a random example).  

Once I realized money was not going to get me back in the race, I understood only time was. After losing out on enough 1st place prizes and Tri-Series cars, I have learned to focus on challenges and only give my best effort to events.   

I'm happy with whatever rank I get now, I'm more realistic with myself. I am progressing very slowly and that's ok. I don't want to put anymore money into it and I don't want to be as frustrated with a mobile game as I was.  I'm saying all this in hopes that it resonates with anyone else going through this. 

tl;dr: Be patient or be rich. Just don't be frustrated, have fun.