Well worst mistake would be to invest too much time and resource to gambling. Let's face it gambling is a game of chance, there's no guarantee in gambling so making a bet that would either make or break you financially and emotionally would be a very big mistake
Something we all regret. I also never gave any importance to my studies as a student due to gambling ( non btc ) and now regret why i did so. Anyways I have a settled life so yeah when I look back, I feel that was my biggest mistake.
At least we are just looking that our past bad experience, as long as we are a changed man now we should have no problem doing gambling again, with the past experience it is expected that we have already learned and we are more mature now in terms of controlling our self.