I don't know if it'll help but I've actually found that most people aren't really that bothered, or at least don't show it. If they ask anything, which is unlikely, I doubt they'd say anything if you just said 'they are self harm scars' or even mention that you have mental health problems. If they have to press on your tummy they may ask if the scars hurt, but that's probably about it.

Remember, whoever it is seeing you has got to keep confidentiality and also you're unlikely to have any contact with them outside of that appointment. Them seeing the scars will not change anything about who you are, your future or anything like that.

Sometimes you just have to just get these things done, they aren't nice, but less than an hour later you are back in your regular life. I tend to book early appointments so I don't ruin the day before hand with it and just keep telling myself, in ... minutes this will be done.