Hi, I don't have experience of DD self harming but she has put a lot of pressure on herself this year as she sets herself very high goals. It has been worrying, exasperating and infuriating at times for DH and I as it really hasn't come from us.
However, she is through it now and we did have a number of meltdowns and tears both during her mocks and actual exams. The best thing I found was to take her somewhere just us two and let her just talk and talk it through, repeating my reassurance of how the grades are not everything. Although she didn't want to hear it, I pointed out how the stress could potentially make her achieve less well if it affects her too much. Tbh this may well be the case in one her her subjects as she did fall apart in the first paper. My reaction to this mini breakdown was to take the afternoon off work, take her out for a drive and some food and just being there, repeating my reassurance that it would all be ok.
I know this is no magic resolution but it works for us, the only thing it does is leave me sad and drained that DD can't see her worth outside exams at that point but things get better quickly and it was really just a case of giving her all my attention for the period she needed someone to fall back on.
I hope your DD manages this next year, does she talk about the stress much? Can you 'prep' her that yes it will be tough, but you will get through it together whatever the outcome? Sure you have already done this but I have had to constantly reiterate this the past few months for my DD.