I don't have a lot of life experience to draw from for this. I may even be younger than you.
I'm sure you've had it drilled into your head again and again not to burn your bridges, keep your options open. That's all fine and dandy but if you love something, don't let those bridges get in your way... burn the fuck out of them if you have to.
Finishing school is a good goal overall, you will have something tangible to show employers, and it will hopefully give you a sense of accomplishment. But do we care if Onstad has a MBA a PHD or is a certified pastor?
If you're going to do a job for 8 hours a day, 5 days a week you need to care about it, it needs to do something special for you.
I'm telling you this because I did finish school. I tried to find something to do with that diploma and it's the shits. I know what I want to be doing, I knew all along, but I went down one of the other bridges and now I'm scurrying to get off.
If you want to be a webcartoonist, take some classes, Study books, movies, theatre, make becoming a webcartoonist your new career. Your mom says no because she worries about your financial situation. She worries about the money spent to get you this far. All of these are reasonable. But if you know deep down what you want to do, the happiness you achieve in the end will make her forget about all that.
If you love kids: teach.
If you love arguing: be a lawyer
If you love music: Sing/play/produce
If you love people: Sell (though not necessarily)
If you love huffing gas: work at a gas station
If you love writing: write (but not commercials. seriously. it's awful)
You can do any of these things... except maybe being a lawyer. That greatly depends on financial situation.
I also recommend persuing these things as hobbies before careers. Get good before you try to make money.