How to stop anxious and insecure thoughts from taking over your mind?

I will get so worked up, even to the point of a full blown anxiety attack over small things.

For example, today I went shopping, thinking I could transfer some money from another account. The problem is, I couldn't get a hold of my dad, who has the account details, to transfer the money. So I had no money and no way home from work. I had to walk. I FREAKED out. Looking back, it was silly, but it felt really bad then.

Also, I get really insecure in relationships over trivial things, like him hearting another girls post of something stupid like that.