I will get to what your therapist really thinks in a minute. But first, it's important to discuss why not holding back is an important ingredient to a successful therapy experience.
There are two different areas where being honest and not holding back is especially important for the therapy process. The first is in describing your feelings and the true extent of them. The reason it's important is that the better a picture of your thought processes and emotions your therapist has, the better they'll be able to figure out how to help you, or what help you might benefit from.
You're probably thinking at this point, "Well, yeah, of course I'm gonna be honest in therapy; why else would I go?". But I believe it is almost every therapist's experience that clients come in and talk about their problems, and they seem to be avoiding going somewhere in their description of events or of feelings. It may be because it's a little scary to talk about our fears, not just on the surface, but the things that truly frighten us even if we feel like we're not supposed to be scared of them because we're "adults". It may be because saying something out loud to another person will make more real the parts of our lives that we feel are missing something, reveal that we haven't lived up to our own expectations of ourselves, or remind us of dashed dreams. Finally, it may be because you think that if the therapist knows what the real reason is (for whatever it is), they will judge you negatively for it. And because of that, the client sort of dances around what the deeper, truer issue is. And it takes an extra 4 (or however many) sessions to get at the real issue.
This is where what your therapist really thinks of you comes in. I'm going to tell you... Get ready. A good therapist is not there to judge you. I will go further, and say that a good therapist will not judge you. It's not that they judge you but won't say it, it's that they empathize with you and will not judge you for it, because they know that we're all humans, and being a human comes with it the capacity to be hurt by things and to struggle with them. That is, after all, being human.
Now, it's not that your therapist won't have some insightful thoughts on what might be happening for you. If you ask your therapist what they think the problem is, they can share with you in a collaborative way what they see as the issues, obstacles, hurdles, and the like. But a good therapist isn't holding back on you thinking "this person really doesn't have their s*** together!"and lying to you about what they think. They wouldn't have made their career and their calling to help people if they did not value being helpful, a whole lot more than snickering at people and gossiping to their friends, and being smug about it. There is a difference between having ideas about what someone's stumbling blocks might be, and thinking less of them for it. A good therapist will never think less of you for the problems you are seeking help with. After all, seeking help shows that you want to get better, and that's admirable in and of itself!
Remember, I said there were two areas where it was important not to hold back. Here is the other. In the business of helping people, sometimes we have to take what people say, and read into it beneath the surface to see if we can uncover something important and meaningful that perhaps the client is having trouble articulating (perhaps due to one of the reasons above). So a therapist might take an educated guess from time to time about what is happening and share it with you in an effort to see if that fits how you're feeling and thinking. We hope that it's right (or close to right), but it doesn't mean that it's always right. If the therapist starts going a direction that you feel isn't helpful or is based on a conjecture that is just missing the mark, it is important to say so, sooner rather than later. Don't hold back! A good therapist won't get offended. After all, they are there to help you, and and good therapist will take your feedback graciously and work with you to find what works for you. (I'd like to add, it's okay if your therapist is an expert and speaks confidently, but if your therapist seems like they care more about saying stuff that sounds impressive than listening to and understanding you, run the other way and get a different therapist!)
On top of all this, saying what you really feel will help avoid any wasted time, so you can make progress faster, improve your quality of life more quickly, and save money because you're not paying for more sessions than you need to! The therapist's office is definitely a place where not holding back is encouraged, and your therapist will thank you for it.