It sounds like you have an excess of stress hormone in your body. The stress hormones such as adrenaline and cortisol are secreted when we perceive situations in a threatening or catastrophic way. So the annoyance of someone walking slower than you may be exaggerated in your mind – the catastrophic negative thought results in secretion of stress hormones, which cause irritation. First of all, recognize this cycle.
Slow deep breathing several times a day can help reduce your body’s stress hormones. Endorphins (happy hormones) such as seratonin are released through deep breathing – this hormone helps you be more patient, and it uplifts mood. Seratonin is also released through aerobic exercise. So try simple slow deep breathing several time a day and try to exercise at least 3 times a week.
Another tool you can try, is to find an affirmation that works for you. A mental statement that you can repeat, that helps put things in perspective. It can be as simple as “I can be compassionate with others”, or “this situation isn’t really that bad”.
I also suggest you get a self-help book called “Mind Over Mood” by Paedesky. It helps you re-assess your thinking patterns, and it helps reduce vulnerability to negative emotions.