Don't quit!!!

This is my take. If they told you that you suck in writing then there is a possibility they are laying the ground work for your eventual firing. If they told you verbally [which is what you say they did] then they simply want you to improve - and they are hoping you do. Actually as someone said, they might actually be grooming you for a promotion. 90% of bosses will never tell you "I am grooming you for this and that...". They simply expose you to opportunities, push you to the max [to see how you would operate under pressure], give you challenging assignments - short notice and all etc. If you crack, there goes the promotion. If you step up, you rise so fast it will blow your mind.

This is what you do. Have a sit down with your boss. And you have to be the one who requests for the meeting this time round! Request him to explain what he feels you haven't been doing right and how he would rather you did it. Explain to him how you are planning to remedy the situation and ask him for his advice including courses you can enroll into if necessary. Follow what you had agreed upon to the letter and after a month or two ask him how you are fairing. You might end up heading the company one day.

Some bosses are mentors while some are just a pain in the... Your boss might be the former. He might just have seen a diamond in the rough and he wants to refine it and you are busy trying to quit!