Hey, i can kinda relate to what you're going through and i'm sorry if this is kinda long, it just really made me remember about my own experience.
I also have an amazing therapist that i really appreciate. She can't make everything better but she's always been there to support me and make my week a tiny bit more bearable. She's the one person i feel like i can tell anything and everything. I really value the trust we built between us and that's why i've always been honest with her. We made a similar promise (more of a pact) than the one you've made with your therapist, a few years ago. When i was decided on ctb last year, i wanted to talk to her about it first, not only to honor our pact, but mostly because i wanted to hear another opinion than the one i've been ruminating over for months, and i did. I'll be honest with you: at first i regretted it. At the end of the session, she was pretty worried about me and told me she had to call my mother so she could take me to the crisis center the next day. I got so scared i tried to OD (bad idea, wasn't even my prefered method at the time) and failed before i finally agreed to go to the center's pychward for a few days. I was angry at myself for speaking about it and ruining my chance at ctb at this time.
But honestly, talking about it with her made me realize i wasn't fully sure about what i wanted. I was in a constant state of panic at the time and was rushing my plan to ctb when i wasn't ready yet (didn't have the chance to see some people one last time, finish writing notes, and get everything needed for the method i wanted to use). I didn't really like the psychward but i met a really nice psychiatrist and nurses that made a lot of efforts for me to feel safe but not confined. For example, after three days inpatient, we decided that i could go back home and that i would meet them everyday for an hour, just to talk, get my daily meds and see where i was at. Anyway, this isn't to tell you that you should always seek help and that you should always aim for recovery, i don't believe that. However i do think that it's really important to think it through, and to me that means not rushing things and being in a state of mind that allows you to make this big decision. And at the time, talking to my therapist about it, getting her point of view on what i was planning, it gave me this kind of reality check and made me realize that i wasn't ready yet. In the end, i'm glad i told her. I respected her trust and it actually helped me.
I don't think it's childish to care about her and the bond you two have. I think it's totally normal to want to tell her the truth and honoring your promise. It shows that you're grateful for what she's doing to try and help.
I don't fully know your situation and how determined and sure you are about carrying out your plan. Sadly i can't put myself in your shoes and tell you what to do. My advice would be to talk to her about it (maybe without too much detail if you're scared that she will lock you up) for two main reasons :
1) through what you wrote, i believe that you do want to tell her because you care, and i think it's really admirable and fulfilling
2) talking with her, she might help you decide if you're truly ready to go, and if you're unsure, please don't rush, you'll always have the opportunity to ctb later if that is still what you wish
I'm not sure if all this will actually help, i'm sorry i'm not so good at giving advice. I'm truly sorry you have to go through so much suffering that you would consider suicide. Whatever you decide, i hope it will help alleviate this burden and that you will be able to feel better soon. I'm not quite sure i'm able to get and send PMs yet (i've just recently started posting) but i would be glad to talk with you if you ever feel the want to! Take care
thank you 
