Digital multiple health behaviour interventions could reduce the prevalence of behavioural risk factors for several non-communicable diseases, but their design could be improved through better understanding of how individuals navigate behavioural change. Using a mixed-methods approach, this study evaluated 2657 prompts that were written by individuals (N = 722) for their personal use while taking part in a digital multiple health behaviour intervention trial. This unique data reflects individuals’ thoughts and feelings whilst actively trying to change, rather than post-hoc descriptions or explanations thereof. Inductive qualitative analysis was used to evaluate the prompts and extract themes within the material; then correspondence analysis was used to test whether these themes manifested differently across four behavioural categories (alcohol consumption, tobacco use, healthy diet, physical activity). Six main themes, each with subthemes, were identified (Encouragement & Motivation, Implementing Change, Self-Focus, Mindset, Negotiation Style, and Specific Motives), which reflect nuances in the concept of self-regulation and align well with behavioural change theory. People favoured different approaches to change depending on the target behavioural category, which were broadly discriminated by dimensions of action vs inaction and external/impersonal vs internal/personal focus. This work reveals the various ways individuals approach and navigate changing their health behaviours. Inclusion of self-authoring modules in digital interventions may offer individuals an opportunity to develop skills and know-how gained through intervention engagement, and remind themselves of personal reasons to change.