How do I stop thinking that my friend is mad at me, or I did something? I know that I'm overthinking the situation, but whenever I think about my friend, I feel really anxious.

The more you understand about something the more clarity you will get about a solution to it. You'll need to firstly understand why you always feel that your friend is upset about you and decide if you have any reason at all to believe that. Though i dont know your exact situation i do understand the worry of what others think, overthinking so much about a situation or what others think often stems from a lack of self confidence or issues with judgement. It could be that you aren't very open or have been judged in the past etc as the brain keeps mechanisms that are meant to protect us but often end up hurting us as they stay when we no longer need them. It could also be an insecurity issue or perfectionism at play as individuals with lack of confidence are often dependent on what others think so do not want anyone to think badly therefore get worried about what others feel. You can never truly know what someone else thinks thats hard to process for people who are worrier about others thoughts. Theres a possibility your friend doesn't reassure you as much as your mentsl state needs or could be hard to read emotionally. It could be their lack of expression persay that makes you generally confused. If you want to stop you will want to recover from why you initially learnt the behaviour and unlearn doing it (you can understand it then begin finding alternatives to that behaviour) i place a lot of value on *learning alternatives* this could be instead of saying sorry say thankyou etc. As well as that it could be a self confidence issue so it could be worth learning to self assure and accept that regardless of what you do you cannot control someones feelings, and thats okay. You must also communicate this with your friend and aim to have an honeat conversation, you may not get far with a lack of communication.