Background: DP & I are unmarried, we've been together 12 years and have three children primary/preschool age.
We're both fairly low earners - DP is employed and earns about £20k and I have been self-employed for the last couple of years and am on track to make about £25k this year.
We rent a 3 bed house privately and would be unlikely to be able to be able to afford to buy a 3/4 bed house locally, though I have a Help to Buy ISA just in my name worth about £18k.
We have a joint bank account and personal accounts but (as far as I was aware!) all our money is joint/pooled.
So, getting to the point - I noticed recently DP had been secretive about banking on his phone. I pressed him about it and he admitted he had an overdraft on his personal account of about £1000. I was upset, but told him I had £1000 put aside for tax and we could use that to pay off the overdraft.
We are (I thought) pretty much debt free (a couple of hundred on store cards that we are quickly paying off) - this is important as some shared ownership houses are being built at the moment in our town and we should be in a very good position to buy one with a 50-75% share. This is the closest we have ever been to buying a house and it is something I have been working hard towards.
Anyway, so as we are discussing the overdraft I ask to look through DP's internet banking so we can work out why he has been overspending and look at a budget together. At this point he is forced to admit he actually has £8k debt! This is a huge, huge amount for us 
I just feel so, so devastated, betrayed and lied to. I don't know where to go from here - whether I should try to help him fix it. Honestly I feel like just walking away at the moment and protecting my money and letting him sort it (or not) himself.
I'm just totally lost on how to move forward and would welcome any advice from people who have been here.