I relapsed back into self harming...?

Relapses happen, don't beat yourself up. I've relapsed many times but you can only try your best and learn from it each time. Think back over the situation and think what else you could try to do next time. You won't disappoint your therapists, they understand that it's a hard thing to get past and by telling them they will be able to help you reflect on what happened and give you support so that you can get back on track. Good luck and don't be too angry at yourself.