Numerous studies have highlighted a range of potential benefits of teletherapy for clients. Nonetheless, researchers have found that many therapists are reluctant to adopt teletherapy in their work practice. There is a dearth of research about how therapists have appropriated telehealth platforms, either to understand teletherapy practice or to understand the challenges and opportunities for system design. The COVID-19 pandemic offers an unfortunate but unique opportunity to learn more about the experiences of therapists who use a range of therapeutic interventions with a range of client populations. In this work, we explore the following research question: in what ways do telehealth platforms support and challenge the work of teletherapy? We present results of semi-structured interviews conducted with 14 mental health therapists during the first six months of the pandemic in the United States. We present a descriptive account of their experiences as well as a discussion of the ways in which the multi-layered and interdependent nature of two facets of therapeutic work---the therapeutic alliance and the therapeutic interventions---made the transition to computer-supported cooperative work particularly challenging. We then offer a suite of design implications for systems that better support the nuanced and unique work of teletherapy.