I've name changed for this as I'm not sure there's a way I can explain the situation with out it being very outing 
My partner has had a gambling problem for most of our relationship, and it's caused us significant problems and the fact we are still together is nothing short of a miracle if I'm honest. He's reached a sort of mile stone of being about 9 months gamble free, which is amazing! And during this time our relationship has improved immensely and I was really looking forward to this Christmas, family time and life just being pretty easy and relaxed generally.
My MIL dropped a bit of a bombshell today which has made me question the relationship and I'm not sure if I'm over reacting.
SD has told her mum that DP left her outside a betting shop, and whilst I'm not sure SDs story completely adds up with timings etc I'm certain this isn't some thing she has completely made up and I'm inclined to believe the bulk of what she has said. This is something that happened quite a while ago now, according to SD, though I suspect it's more recent than she has said. Me and DP have a DD together and I'm certain if I knew at the time he had done this that I would have left him, it would have been the final straw.
Given that things have been improved and his gambling addiction is currently under control, I'm not sure how to handle this? My instinct is to completely flip out and act (I think) quite irrationally, I'm really hurt mainly because I found out not from DP but from his mum. I feel a bit of an idiot being the last to know and him not mentioning it to me. I honestly arnt sure if I can be with some one who has ever at any point in their lives feel it's acceptable to leave the primary age child outside a betting shop. I've brought it up with DP, who is in typical DP fashion minimising, and I think lying to lessen the severity of the incident, and making out I'm over reacting over some thing which happened in the past.
I'm not even sure what I want anyone to say right now, I think I just need to type things out and think them through perhaps? 