How do I build an exercise routine I actually keep?

Start with something embarrassingly small. If you think you should work out 5 days a week for an hour, start with 3 days for 20 minutes. The biggest reason people quit is they set ambitious goals, miss a few sessions, and feel like failures. A small routine you actually do beats a perfect routine you abandon. Attach it to something you already do, like working out right after work or every morning before showering. Same time, same place removes the daily decision. Also pick activities you at least tolerate, not ones you think you should do. Track your streak but give yourself one free pass per week without guilt. Consistency over months matters far more than intensity in any single session.