A list of qualities that your Mary-Sue character might have. You don't need all of these, but it's a good place to start, so you know how to change. Also, one or two things from this list doesn't make the character a Mary-Sue. You'll have to judge that on your own!
Maybe her first name is your middle name — or the other way around. Maybe her name just sounds like yours. Either way, this is a great sign of a self-insert Mary Sue.
Of course, this draws people to her. She's not going to have any noticeable flaws, even if she thinks she does. They aren't going to be a big deal at all.
She has different coloured eyes. This might be to show her power anime-style, or because she's a mix between two (or more) different powerful races. It might also be because you just want her to look cool.
Her abilities set her apart from everyone else. They might also make her the only fit for some job — like saving the world.
She can't just have a normal love interest. The love interest needs to be the best person on offer and a complete catch. They might be a bad boy or have flaws, but everyone else is going to be jealous of the two of them.
She is persecuted for some reason. Maybe it's because she's part of some persecuted race. Maybe it's mean parents who treat her badly. It's definitely not her fault, though. She's too nice and pretty and good to do anything wrong.
Maybe it's because you don't think she does anything wrong. Maybe it's because anyone who calls her out is portrayed as "mean" or "bad", so she doesn't need to listen to their criticisms.
If they don't like her, it's because they're jealous or evil. Either way, anyone who doesn't like her will pay by the end of the story. Or they'll change their minds. Or both.
She might not notice this, but people around her will compete, fight or argue for the chance to be with her.
She might have been raised as a "peasant", but secretly her parents are rich and in charge.
The prophecy says that she's the only one who can save the world. And no one else can fill her role in the prophecy! If there is anyone else out there who could do it, they're never mentioned.
If they ever mention religious views or say anything political, it's going to fall in line with what you believe.
She reveals that she has this awful, dark past where people treat her badly, but we never once see her overcoming PTSD, getting flashbacks or even having her world view shaped by her past experiences. She might say that she's affected by it, but there's never any real proof.
She dies to save someone or for some other noble cause that makes her seem like an incredible person, but she doesn't stay dead for long. She is resurrected pretty soon.
There isn't a moment in your story where your character questions if her actions are right. If she does question if they're right or wrong, it doesn't matter because they were right anyway.
She just so happens to be very talented at the one thing that everyone needs her to be
You don't really see the flaws stop her from doing a lot of things or mess her up in any way. She may have flaws and they may "stop" her a little, but the impact isn't huge and she can get on her feet again very quickly.
She's too nice. Too kind. Too friendly. Things that would be good qualities in moderation.
She never really fails in the story. People just get in her way and mess things up for her. It's never her fault.
If you ever see the Mary-Sue fail, it's not going to last very long. She's going to bounce back very quickly and get better straight away.
It doesn't matter if she doesn't have much experience. She's going to naturally be gifted at a lot of things. Chances are that she's going to beat people with loads of experience. Beginners luck, maybe?