COVID-19 shuttered schools across the United States, upending traditional approaches to education. We examine teachers' experiences during emergency remote teaching in the spring of 2020 using responses to a working conditions survey from a sample of 7,841 teachers across 206 schools and 9 states. Teachers reported a range of challenges related to engaging students in remote learning and balancing their professional and personal responsibilities. Teachers in high-poverty and majority Black schools perceived these challenges to be the most severe, suggesting the pandemic further increased existing educational inequities. Using data from both pre-post and retrospective surveys, we find that the pandemic and pivot to emergency remote teaching resulted in a sudden, large drop in teachers' sense of success. We also demonstrate how supportive working conditions in schools played a critical role in helping teachers to sustain their sense of success. Teachers who could depend on their district and school-based leadership for strong communication, targeted training, meaningful collaboration, fair expectations, and recognition of their efforts were least likely to experience declines in their sense of success.