Is it really believable? I don't think someone to lose that amount, nobody does in just one session.
Besides, you have noticed that OP hasn't complained about his losses, and never felt regret or sad about the results as it was easy to make a story that never happened in real life.
Anyway, losses are usual in gambling and if we can't control our expensing habit, losing more is very possible. A reason why we should have set limits on the amount that we can spend in one day.
We do not really have a way to know and personally I would prefer that it was not real, since in my case it would really pain me to lose so much money over such a short span of time, however we have seen people like Drake losing way more money than that before, with the difference he is incredibly wealthy, but this makes me think that even if the story of the OP was not true, there are probably many gamblers out there which have experimented those kind of losses before.
There is not a single gambler who has never experienced defeat, defeat is something that is bound to happen. The size of the loss depends on how big a deposit we make and whether we like to chase losses or not. I never deposit much so usually I will spend around $50 in one session, if I get emotional sometimes I will chase my losses and deposit another $50. If I have made two deposits and still lose then I will stop and gamble again next week