Is it time to quit my day job?

Personally I would stay with them at least through the summer so you leave on good terms, also be really sure your freelance work isn't just seasonal as in is it likely to be as lucrative all year round or does the type of writing you do have more active times of the year. You sound like your heart wants out but your head is not ready to take the step. I would stick with your head for now and hang in there a little longer and do your math and list all your worst case scenarios and look at how you would deal with them. When you can answer these, you're probably ready to make the move.