The current study explores the learning processes occurring in online discussion forums, from a perspective of self-regulation learning which facilitates in planning, monitoring and reflecting on learning. Participants were 47 teachers, who took part in a 2-year mathematics teachers professional development program. The study was conducted in two stages. In the first stage the teachers acted as learners through collaborative solution of complex mathematical problems in small groups, while in the second stage they led collaborative problem solving in forums as mentors. The study showed that problem solving forums have a high potential for developing teachers' own self-regulation skills and increasing their effectiveness in collaborative problem solving. This experience is reflected in their pedagogical activities, aimed at developing self-regulation skills of their students. It also empowers teachers to support students in solving complex mathematical problems.