You did exactly the right thing, Jessica. If there's one person you should be absolutely honest with, it's your therapist. He or she is (or should be) someone who looks at you from a neutral, unbiased position and can help you through this without any preconceptions or emotional attachment. Friends, family - they're useful to talk to, but they do have a nasty habit of steering you towards what
they want you to be, not what
you want to be. Just look at the number of stories of parents and other family members who disown trans children and siblings unless they repent and stop being trans.

Because the therapist is someone who is neutral, you really have nothing to hide. The worst thing you could do is pretend that you're something you're not - therapists aren't mind readers, and if you go into therapy pretending that you're 100% sure, the therapist might wrongly assume that you're more gung-ho about transition than you really are.
I doubt seriously any legitimate gender therapist would hold you to what you wrote on that initial intake form. The purpose of therapy is to work through issues, to develop as a person, and if that development is that you're committed to transitioning after all, then so be it.
My own therapist has told me that he is most comfortable with clients who are still unsure - it's a good indicator that the client understands already the gravity of transition. It's no small process, and it's not to be taken lightly. Clients who come in basically asking for a letter, he's more concerned about because they might be rushing into something that they really haven't worked through properly. Generalizations, but illustrative of the fact that uncertainty about transition is a good thing, not a bad thing.
I'd also suggest trying to pin down your uncertainty. Are you uncertain about being trans, or are you uncertain about whether or not you want to transition? There's a huge difference. For me, I'm 100% certain I'm trans, but I'm not 100% certain I want to transition. If there was a way of stabilizing my mind and becoming content with my body without transition, then I'd take it in a second. The process of transitioning is an immense undertaking, largely irreversible, and there are many trans people who are trans inside their heads but take no (or few) steps to actually transition. It works for them.
But to answer your question - finally - I fully agree with the other posters in this thread. No good therapist will withhold endocrinology referral letters based upon your initial uncertainty. If, after enough therapy, you come to the conclusion that you do want to transition, then you'll have no trouble getting that letter. The therapist isn't there to rubber stamp letters for every person who walks in the door and proclaims that they are trans. The therapist is there to work precisely with people like you, who are questioning things, and help them to come to the right decision for their unique circumstances.
On a side note,
enjoy the therapy. It'll have its ups and downs, its uncomfortable questions, and its moments of uplifting revelation. It's part of transitioning in its own way. Open up to the therapist, be brutally honest, because he or she is not judging you. It really is an amazing journey, and you should look forward to each and every session, tears and laughter and frustration and all.