What has mental health issues got to do with drink driving? Drinking yes, driving no.
I've had mental health issues all my adult life, I've suffered some extreme stress and loss this last year, I've drunk myself silly over it....I didn't drive while inebriated
People make very bad decisions while suffering mental health issues, people make bad decisions due to alcohol, just because you didn't doesn't mean that it's not a possibility. You only have to look at suicide as a prime example of this. It's a bad decision made at a moment in life where the person is at one of the lowest points possible and chances are, if they could hold on longer they'd pass through that feeling and be thankful they didn't make it.
As for the situation with Ant, I'm not a fan of the pair and he's made a completely stupid decision for someone so in the public eye, but, that doesn't mean it's the end of anything. Look at Robert Downey Jr and his history with drugs/prison. Celebrities can and do turn their lives around.
I personally wish him all the best in his recovery and hope that he gets and stays clean, that he makes it through the bad times. No doubt once he is clean he'll be making amends with the little girl and the people involved in the accident.