The whole point of these kinds of 'promises' is to assist you in stopping a destructive behavior. There really is NO other purpose to them. I know that having made such a pact with a T is one of the factors that assists me in abstaining from SH. However, if T is pushing this to a level which is totally unrealistic then I think its going to exacerbate the situation. Feeling bad because you have let him down - or feeling like you need to keep info from him - is only going to make the situation worse. Could you change the promise to that you will message him first if you are going to self harm? Rather than that you won't self-harm? Perhaps you could also discuss what it would mean if you were not able to keep your promise. What would it mean for the relationship?