I also have bipolar disorder (type I, diagnosed at 17). Mine is well controlled with medication; I haven't had an episode (except for minor depression related to school) in over a year. My therapist is a general therapist and she understands that having bipolar disorder is separate from being trans.
That being said, if you are unstable (frequently having either a manic or depressive episode) or off medication (when you should be on it), there might be a delay in starting T. Unless gender dysphoria is a major trigger which would be improved by T, then your therapist will likely want you to be stable before starting hormones.
If you are stable, then I see no reason why you wouldn't get your letter on "schedule."