Probably deserves it's own post but a large part of things like this happening is 'tomorrow' As in "I have to take care of that, I should have some time tomorrow." And you don't get to it, and it's the next day and the next day and so on.
That's true! I'm guilty of this myself (pretty important stuff backup-wise that I need to do 'tomorrow'). Nobody's perfect; just be aware of how risky (or not) your seed backup storage is and in which cases you're safe & which you're not.
Like, how is it possible that someone has access to all your accounts after you lose that thing?
One possibility is that the phone was stolen by someone who knew exactly what was on it, which again would mean that it was a person close to the OP or someone to whom he was bragging about his gambling successes. OP should make a list of those who knew and try to find the possible culprit using the method of elimination.
In any case, it's stupid to give up, because someone must have seen something and knows something.
Good point; in that case, @OP it means there is no way to recover your funds except recovering that phone itself. Sounds like there might be a chance. In case it's an Apple device, it should be trivial to track its current location, by the way. Other manufacturers like Samsung have similar technology for tracking and locking your devices.