Before you start writing the most tragic back story ever for your character, I’d like to point out that not all good characters need angst. Its best if used sparingly and not the main focus of the plot. Also, just because someone’s OC has an angsty past, does not immediately make said character a Mary-Sue. If they have angst for all the wrong reasons, then they are a Mary-Sue.
If you are writing certain types of trauma (rape, child abuse, ect.) try to include a trigger warning, unless it is a fanfiction of something that possesses the same exact type of trauma.
Part one: Writing Angst in General
First decide why you want to give this character angst in the first place. If you don’t what your character to be seen as a Mary-Sue, than you must have a good reason for their suffering. A good reason for angst might be to show why your character is the way they are today. Some bad reasons for trauma would be to make people feel sorry for your character, o