Right. Let's look at what she actually said, shall we?
I'm so ashamed.
A long tiring day and the inlaws visit this evening. MIL is true to form, rude, doesnt even look at the lovely gifts we bought her. Without going in to too much detail, the family are very close and I began to feel my usual inferiority around them all. Left out, disregarded, not listened to.
I decided to go out for a breath of fresh air feeling downhearted this, to me, reads like 'I went off in a sulk because they were all having a nice time together and I felt jealous' came home to see they had left, DH asked what my problem was. I told him honestly, he tells me it's all in my head as usual and then I suddenly slapped him and walked off.
I've shocked him and myself, I've never done anything like this before.
Good. So she understands the gravity of what she did and she sounds genuinely remorseful. But then: I'm massively frustrated at DH not being able to just accept and acknowledge my feelings have been for a while but never expected I'd do anything like this. What shall I do?
she attempts to justify it, by turning the blame for her loss of control onto him, because he doesn't understand how she feels.
DH hasn't said a word and has gone to the spare room for the night. I feel like I've ruined Christmas. I was looking forward to a lovely relaxing evening with DH once they left. I'm lying here in tears.
So she resents the PILS monopolising her DP and pushing her out, as she sees it, and she can't wait to have him all to herself again. She really expects that she can walk out in the middle of their visit (which was only for a couple of hours in the early evening anyway by the sounds of things) and stay out until they have left, for no good reason other than that they are annoying her by being close and not paying enough attention to her. And then she expects to be able to walk back in and for her partner to cuddle up and relax like nothing's just happened. And then when he has the audacity to ask what her problem is she slaps him. 
There is an alarming lack of awareness there IMHO, and zero regard for how she's just embarrassed her DH in front of his parents and put him in a really awkward situation, left trying to explain her behaviour. It's all just ME ME ME, woe is ME listen to how I FEEL.
DH and I have talked and he has been very forgiving, telling me that he's aware of how frustrating he can be and that provided it doesn't happen again, this one off incident can be forgotten.
Aah, good. It would appear that her DP is giving her a get out of jail free card and allowing the OP to absolve herself of some of the blame for hitting him, by acknowledging that he can be can be very frustrating to argue with. So that's alright then. 
-Not sure I feel too comfortable with the outcome. Surely there are issues to address here. Nothing like this has ever happened in our relationship before and we are very loving most of the time. I understand that I need to take full responsibility and I do, I'm mortified. It was a split second reaction to pent up frustration and I should have had better control. I don't thinking forgetting it is the answer._
All good here. I don't think the OP is someone who is likely to turn into a serial husband beater, nor did I ever say that. It's quite likely she will never do this again and it was a one-off loss of control. It happens. I'm more concerned with why she has so many issues with so many people in the first place.
This is not an excuse, more of a background to what happened today. But the frustration I feel in response to DH's lack of regard for how I feel around PILS has reached optimum levels lately. I've no idea how to knock the door down to get him to hear me. I don't want to go into masses of detail but they have genuinely caused problems for us for a number of years, to the point I now struggle hugely in their company which causes the inferiority etc. DH is a stubborn man who fails to see anything he doesn't want to see.
No. He hears you and he sees it. He is just worn down by your insistence that his mother is the problem. He thinks that the main issue here is your jealousy and your feelings of insecurity, inferiority and persecution. Because that is how you are generally, not just with his mother.
Look, I don't know you, or her, but I am getting strong vibes from all of your previous posts that your DP has a point. You never say what she does that is so wrong, just how she makes you feel. You always say you don't want to go into detail, but I suspect you would be happy to go into detail if only you could put your finger on it about what it is she does that makes you feel so shit about yourself. The fact that you can't says much more about you than it does about her. And it isn't just her, is it? It's anyone who takes your DPs attention away from you.
I never want to behave that way again, but I know that foremost, I need to address the frustration. I am already in therapy dealing with some childhood issues so can address the frustration issue there.
Good, so carry on with the therapy, work on understanding why you feel this way and try not to take your frustrations out on your DP.