Unfortunately self-harm doesn't discriminate. Your already having a hard enough time of it as it is don't add to it by beating yourself up telling yourself that your too old to self-harm and you should be over this by now.
What you need to do is to remind yourself that you have been in tough places before and you have got through them so have a little faith in yourself that you can get to that place again. Hopefully the med change will help, sometimes every now and again we do need a bit of a med tweaking.
Do you have any other coping strategies you use when things get tough? If not this might be something worthwhile investing a bit of time in. Hang in there lovely, it will get better