Hello there!

I do not have any idea if this will help you, but here it is anyway.

Before you get into the character's proper backstory I'd recommend you to visualize smaller, fundamental aspects of what type of character you want to play. And by this I mean to start with basic knowledge about his/her person. What's their alignment, what's their moral code, what are their goals, motifs, skills, ideals, flaws, strengths, weaknesses, appearance (if it can help with their personality) and so on. Start with a few basic personality traits and then expand your vision of the character into a backstory. Basically, start looking at small details that will form the bigger picture instead of looking at the bigger picture searching for small details. If this actually makes sense.

Once you determined a few aspects and principles for your character you may start writing the proper backstory, which is usually easier said than done. Here you can take inspiration from works of fiction you enjoy. Games, books, movies, TV Shows, Anime, D&D sessions, Graphic Novels; everything is fair game. Remember to include in your writing aspects and events that are meaningful. Try to write in such a manner that the backstory will explain who is your character, what are his goals, how does he act to achieve said goals, what are his motifs and everything you deem worthy including. The purpose of the backstory is to give you, the player, a baseline for your actions and decisions. It's important to have a background for your character that will reflect onto the in-game journey.

I do have a few examples of character backstories if you're interested to read them. Two of them are the ones that inspired me to write short stories for my characters and one is the last character I've played with. Here they are: backstory 1, backstory 2, backstory 3. The last one is mine. If you have problems with the alignment you'll have to download the file. Also, here is the artwork that inspired my character's appearance.