Guided by Dynamic Systems Theory (DST) and multidimensional perspectives on individual differences (IDs) in instructed second language (L2) learning (e.g., Larsen-Freeman, 2014; Sun & Zhang, 2020), this longitudinal mixed-methods study focuses on three conceptually different categories of L2 learners' IDs: working memory (WM), motivation, and the amount of L2 exposure. Despite previous propositions regarding cognitive, affective, and exposure-related IDs being interrelated (e.g., Pawlak, 2012), including WM and motivation (e.g., Serafini, 2017) and WM and the amount of L2 exposure (e.g., Denhovska et al., 2016), the longitudinal development of motivation and L2 exposure, as well as the effects of WM, motivation, and L2 exposure on learning gains in different skills and at different stages of L2 development have not yet been investigated. In this study, I aimed to address this gap and explore how L2 learners' motivation and the amount of L2 exposure change over time and how WM, motivation, and the amount of L2 exposure affect learning gains in lexicogrammar, speaking, and writing at different initial proficiency levels. Considering that Second Language Acquisition (SLA) and Instructed SLA (ISLA) research are dominated by studies of more commonly taught languages, I also strove to promote the need for more ecologically valid research with Less Commonly Taught Languages (LCTLs) and investigated IDs in the instructed L2 learning of Russian. The participants were 52 students recruited from an eight-week Russian summer immersion program from four curricular levels (First-Year, Second-Year, Third-Year, and Fourth-Year Russian). The program was conducted online due to the COVID-19 pandemic. The results supported the view of L2 learners' motivation and L2 exposure as constructs showing stable and dynamic patterns and that different constituents of WM, motivation, L2 Russian exposure, and learning gains were engaged in complex relationships that varied at different curricular levels, language skills, and phases of the study. This dissertation includes a discussion of theoretical implications with regards to DST and the mixed-methods approach as future directions for ISLA studies of IDs, including research with LCTLs. The dissertation also includes a section on practical implications that discusses pedagogical aspects and implications for the development of background questionnaires and placement testing. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]