How do I convince myself that I will never be able to start a romantic relationship with my therapist? I let go of her but I still fantasize about adding her on Facebook after some time.

It's pretty normal to have crushes on people, even those in a professional relationship with you. It sounds like you have a strong attraction to your therapist. If it continues for over a month or so, I would suggest seeking some outside support on this - perhaps talking to a trusted friend or family member, to see what they think, or even another therapist or counselor. Seeing a new therapist who you are not attracted to may end up being best. Perhaps you can explain to her how you feel, or at least that you're uncomfortable with her as your therapist, and perhaps she can refer you to someone else. However, if you feel like she is truly helping you and would be the best therapist for you, and you can keep your romantic feelings towards her to yourself and not act on them or have them affect your sessions with her, then keeping her as your therapist could be a possibility.

In my experience, it's sometimes better to try to change my relationship to troubling or irrational thoughts rather than argue with them. 7 Cups has a free self-help guide to Acceptance and Commitment Therapy, which has techniques for creating distance between yourself and difficult thoughts. You learn skills for what is called "defusion" – basically realizing that we are not the thoughts in our heads, and that we can continue to move toward the things we value even when we have them (much as a bus driver can continue driving toward their next stop even if passengers are being rowdy in the background).