I wouldn't suggest you to quit. In fact, try to evaluate your strategy and see where you went wrong with your domain investing. Making the most out of domains is not a piece of cake these days and it takes knowledge, insight and skills to be successful.
I have been investing in domains since 2009. I stopped for a while and then restarted in 2014. I am not an active domainer but I also wouldn't call myself a domain flipper (selling domains for fast cash). Instead, I am more like domain investor with long term goals. I intend to invest in domains that I find of quality and for which I can wait till I get the right price. I sold my first domain in 2015 at BrandBucket for $2,595. Sold my second domain via inbound inquiry this year for $15,000. That's right, I have sold only two domains so far but I am happy with the results considering my investment and profit margin. I have a portfolio of just over 100 domains; all dot coms. I have tried outbound for couple of geo domains but never had any success. So I am now more focused on marketing my domains to medium businesses instead of small ones.
The point of sharing my story is to tell you that if you have the knowledge, skills and insight, and you know what you are doing, you will certainly win; sooner or later. Just be consistent with what you are doing.
I am not saying that I am an expert domain investor and you should listen to me, just wanted to motivate a fellow member who is near to giving up.
So my advice to you is, never give up, stand up and keep trying.