Don't sell your apartment. Pressumably you want to take the equity to pay for training and rent somwhere to live until you get a job flying?
Is there no way you could move out of your apartment and rent it to someone else (so they can pay off the rest of yoour mortgage), and then rent somewhere cheap enough to allow yourself to be able to pay for your training?
I'm 26 with a PPL and started ATPL distance learning just before Christmas. I don't own a property, but rent and pay for my training out of my salary as I go. I love flying and would be ok to never fly commercially if I couldn't find a decent job, though I do want to go as far as CPL IR atleast, if only for flying to/from where I want, when I want.
I can't help but think you're making a bad decision to sell up and reinvest the money in training for a job that you may never get. If you keep your current investment and pay for your training over a period of a few years, you may find jobs are easier to come by when you finish, and you won't have lost your capital, and be out of work during training paying over the odds on a gold-plated integrated course.
Think about what you'd do when you have your fATPL and no one is replying to your job applications? Would you rather be broke and trying to get your old job back, or would you rather still have your current job and be a few years behind (with an apartment investment)?