First, I just wanted to say wow that's really shitty regardless of who took it, and I imagine you are feeling a crushing sense of disappointment, shock and hopelessness. I felt that when I was poor and lost $100, and losing what you lost is so much worse.

Now with that said, you absolutely can come back from this. You have already demonstrated that you can manage your money (choice of where to store it notwithstanding) without blowing it all, and you plan to invest in an education and self-improvement. This may feel like a dark, dark time to you, but I hope you will keep trying anyways, because if you do I promise you will persevere.

I sincerely hope that in 10 or 20 years time when there is another major recession you will look at $500k losses that you have in your investments (because that's what happens during recessions) and look back at this time and say "man, remember when losing $15k felt like the end of the world?". And then you will shrug and go on with your life because even though you just lost $500k, you will know you will recover and you can deal with this kind of thing.