A therapist is supposed to:
-Help you make sure of your gender identity, if applicable. (This is where most of the bad ones fail)
-Make sure you don't have other issues that might either cloud your judgement or put you at risk during transition and, if applicable, treat them at least to a point where there is no major worry.
-Make sure you are ready for transition and understand what it entails, and give you tools and resources in this goal.
-Help you determine the appropriate path and course of action for your transition.
-Refer you for the next necessary steps in transition, if the therapist has the ability to do such a thing.
-Accompany you through transition, if desired, to support you and answer your questions.
-Provide legal support later if, for example, you need a letter for SRS.
Good therapists should do that, and should not judge you. The people you've talked to visibly just got unlucky. Seeing a bad therapist is useless, but that's not true for all therapists.
As for how you should act, well, be yourself, be honest, and be straightforward (but not aggressive) regarding what you want from the therapist. Do care about what you feel like to them (if you want HRT, appearing fragile, fetishist, suicidal and instable when you aren't really isn't the best plan), but don't hide or stretch truth. If seeing a good therapist can be useless, it has to be when you lie.