It is hard... have you had any problems with confidence, depression, stress before? Sometimes things like this can make us very irrational. I don't often say this as I don't like it but I am a Manic depressive and this drives me to do the most irrational things. (I am not for one second saying that you are) but from my experience I know I am jealous of everyone who go's near my B/F even his sister and it drives me insane. I too have been for hypnotherapy and it worked wonders - I am not saying I am cured butit helps so much. I have a tape to listen too everynight b4 I sleep and it calms me and makes me feel safe and secure. What sort of Hypnotherapy did you have? - I just slept in my sessions - no asking of questions or probing about in my mind. Just a relaxtion thing, maybe it would help to try this?
Does your B/F show you enough affection? - maybe that is the problem?
The easy thing to say is don't let it get to you and ignore the feelings but if you feel it is as bad as that then maybe you need to talk to someone. But somone different. Not all therapists are the same and they don't all have the same technique. I feel I have babbled and I apologise. But I hope you get this sorted soon.