Struggling with Saying No and Dealing with the Consequences

guys help me. I am going through some tough emotions over something I did earlier in the day. it has something to do with Me not being able to say no to someone and make them wait. I just am really upset over it like I am literally crying right now. I just wish I can say no to more people and things. that Is one of the problems with what I did today that really pissed my dad off. I understand the fact that what I did was wrong and that the person i helped did say s*** about My dad more times than once but I was helping an animal not the person. he expected me to wait until he got home. he would not have found out If I had kept the main door of his shed locked. then I probably would not be in this mess. I just need someone to talk to right now. I am still crying pretty good and i need someone to reassure me that everything is gonna be fine for a while after what happened today. HELP!