Regrettable describes things that make you feel sorry. Letting your sister borrow your own snow shovel became a regrettable decision the moment the big storm started.
Actions and events are regrettable if they result in feelings of regret, or if they're unwelcome or undesirable. Wearing your least comfortable shoes on a walking tour of Paris will be a regrettable choice if blisters form on your toes. Remember, regrettable describes things like decisions, actions or words. Regretful is for describing people and feelings that express regret.