What advice do you have for managing irrational thoughts and feelings?

I have a good friend who is a very reasonable person, who sometimes gets irrationally upset about things. For instance, sometimes she’ll be reminded of a minor conflict with her boyfriend that happened over a year ago, and suddenly find herself angry about it all over again.

Lately, I have been having irrational insecurities about a relationship. They really have no basis in reality and I know I am being unreasonable, yet I still have very strong feelings of anxiety over this.

We both think of ourselves as pretty logical people, yet these feelings of anger, hurt, worry, etc. can be pretty powerful and are tough (maybe impossible?) to reason away. How do you manage problems like this when they happen to you?