I have the answer to this, for sure.
It's absolutely, one of the most difficult things to deal with in therapy. I am speaking in terms of, from the clients end.
Is when there are co-occurring transference's happening.
Transference, is basically when the roles of someone get passed to therapist, or client, in the form of specific feelings.
Is very difficult to deal with. I should know, as I am dealing with it currently, and will not discuss any further details because it's personal, but I will say, that it's not easy to process, because transference in it's general nature, is somewhat mysterious, and is a fabrication of a fantasy that we create, based on what we want. Generally not backed by anything to support the fantasy, as therapists don't disclose much personal information.
For many people, it is uncomfortable to tell others that they go to therapy, or accept that they need it, because there is still the stigma that "those who go to therapy are crazy."
In therapy you should not feel uncomfortable, a good therapist will seek to foster trust and communication in the therapeutic relationship, so you can open up without problems, you should also emphasize that you will not judge and respect you so that you feel comfortable, if you see that is not possible for reasons of your own (difficulty to fit yourself, you remind someone, etc.), the ethical thing is to recommend you with another professional.
There are techniques dedicated to close cycles and handle emotions grounded in trauma, there you can feel discomfort too, but your therapist should help you to generate skills to handle that discomfort and overcome it, along with the negative emotions associated with those cycles, traumas and more.
In general, a good attitude towards therapy and a well-worked therapeutic relationship help to avoid or reduce discomfort to a minimum.