I've decided to give him a second chance
well, it's not a second chance is it? It's at least a third or fourth chance.
because I know he's truly sorry for what he did and I don't think he will do it again
Why don't you think that? Every other time you asked him to stop he said that he would and then didn't. So isn't it extremely likely that he will do the same this time?
He does truly hate what he has done to us both and after seriously having a long argument last night...
If he's so remorseful, then why was there an argument? Or did he try to minimise, dismiss and shout you down, and only when that didn't work, switch to "Oh my god, you're right, I see it all so clearly now"?
I know he didn't intentionally set out to hurt me
No, he just doesn't care if he hurts you AND YOUR CHILD or not.
if he does it again it's over for good he knows this and there's no going back
I bet you said that last time. And the time before that. So actually, rather than "knowing" if he does it again he's out, he can be fairly sure that WHEN he does it again that all that will happen is you'll have another go at him.
He says he'll go for help if need be but he wants to do it off his own back
Or "Don't expect me actually to stick to my word by contacting a drugs counsellor/talking to NA/visiting my GP, etc. I'm not doing to actually DO anything, because I'm not taking this seriously."
I'm still hurt and will take lots of time to rebuild my trust and I hope it's worth it
OP, you are so desperate to believe him - desperate for everything to be OK, for this to be magically sorted out, for this to be the last time and for him to somehow turn into the loving, reliable, straight-up husband-and-father you're trying so hard to pretend he is.
I hope you are still reading this - I don't imagine you will be back to comment but I hope that you do read and re-read it, and see that, underneath the frustration some posters are displaying, is a genuine care for women who are in the situation you describe.
It's like watching a car hit a brick wall, and seeing the driver getting out all dazed and hurt. Then after a bit, she gets back into the car and drives straight back at the wall again. Except you've got a kid in a carseat next to you.
Please think hard about what you want for yourself and your DC.