Losing patience and anger with customer service

I’ve been told many times I’m a patient person, and I think I’m kind and polite, but one pattern that has emerged that I’m troubled and ashamed of is how quickly this disappears with customer service issues. I think years of building frustration with companies has led to this, but I know the person I’m dealing with is not the person responsible, so why do I get impatient and angry and just not that nice?? Sometimes I’m very patient and understanding, but sometimes I’m not.

I just want to handle it in a more mature, even-tempered and respectful manner, living up to the standards I have for how I behave and treat people, and I’m at a loss how to break this pattern because it seems to all happen so quickly. Any advice on how to get a break from the pull of the strong emotions and behave differently?