to the bitter end: o que isso significa?

For those who do not know, I have this hobby of drawing all the time. It's a habit I've had since my early childhood when I grabbed everything I could get my hands on to draw what was on my mind. Today, I try to draw with more details and attention, which most times takes way more time than what it used to before. Sometimes when I'm in the middle of a very difficult drawing, people call me to do something like have dinner or take a walk outside, but I say: I'm staying here until the bitter end. What does that mean?

It means I'm going to continue to draw until it is finished, although it is difficult and it may take a while to finish. When I was in college, I used to be like that. When we received new projects or works to do, I would stick to it until the bitter end because I wanted to be free from the obligation as soon as possible. That's all. Take care!