The psych people I've talked to explain it as a rule of thumb "If it's not a problem, it's not a problem".
Say you feel you need to wear a hat made of marshmallow. Is that a problem? Well, is it? For example if you lost your job because of the hat, that's a problem. If you spend so much on marshmallow hats that you can't afford your rent, that's a problem. If you killed somebody's dog because it tried to eat your hat, that's a problem, and so on.
If you spend money on MTG and enjoy it even if it's $5000 per week that could be fine, psychologically at least, if it didn't cause you or others to suffer. But if you find you're ruining your life, or other people's to buy cards, even if it's just $50 of cards per month then that's a gambling problem.