How to Stop Writing With Having A "Wall" Between You and Your Character.

I normally write in third person limited. However, I want to try to learn to write in first POV, even if it's just to have the option of writing in first POV. There's just one problem that I've always encountered whenever I tried first: I feel like my characters sound more like me than they're supposed to. Instead of doing or saying things they would, they're doing what I would do. This, so far, only seems to really happen in first POV. In third limited, where I have some "distance", I'm able to distinguish what I would do and what my character would do. It's like this "wall" or some kind of small distance between myself or my character is necessary, and it's been like that for years now. Without that wall, my character ends up sounding more like me than s/he should. I really don't how to get rid of it honestly, and how to get better at first person.

Does anyone have any suggestions?

At least for me, I've had a first person character drift into me a bit after an extended period of writing. Usually, I can get into character just fine for, absolute minimum, a few hundred words without a problem. I start with a general feel for what I want the narration to be like, and then I write those first few hundred words like that. Doesn't matter what I'm writing or even if it's useable for anything.

Then, once I have that short sample down, I start looking for quirks. I know I have my own habits when it comes to writing, especially informally: Favored words, sentence constructions, phrases, etc. So I look for anything like that, or anything that I could turn into that, within the sample. Then I write a couple thousand words where I'm consciously using the chosen quirks and aggressively watching for and editing out my own. This doesn't make for natural writing or very good flow, but once I have the style down I find it much easier to slip into character for more extended passages without having to think about it any longer.

Maybe you can learn to let your character be an individual even when there is no wall.

I've heard it said that your characters are always a part of you. I don't mean that in the sense of "I love you you're a part of me" but that you take some part of your own psyche and make it into a new character, and the important thing is to take only the one part. (And of course you add things that aren't you at all, but the idea is that the core of the character is from you.) Someone wrote that Shakespeare's genius was that he did this with all characters, even the worst villains, that everyone has bits of evil (will to power, selfishness etc) in them and he just took his and magnified them into his villains.

To bring it down more to our own level, I wrote my first book from the third-person POV of a boy and I'm writing my current one from the first-person POV of his sister. The boy was more like me--a lot of imagination, often saw images in his mind, took everything hard & felt it deeply, worried about being accepted, etc. The girl is more different from me--she's very practical, lives life more through her senses than her mind, impatient, cheerful and (over)confident, takes things in stride more easily. But I have a side of me that is like that, that I am sort of starting to explore more. A side of me that gets tired of thinking and wants to go out and get something *done,* split some wood, weed the garden.

So when I write the girl's POV, I try to connect with that side of myself.

So maybe you could do that. Identify what parts of yourself are in your character, and instead of trying to keep yourself out of your character, just make sure and center yourself into the part of yourself that *is* your character, and write from that.

In a word: rewriting. With each layer of rewriting, you can push the character's phrases and thoughts further from your own.

My amateur approach:
Did you ever roleplay?
After many years of playing different characters I do know that it's extremely different to get "in" character and to stay "in" character, and I'm only talking about my own characters here (I never fan-roleplayed anything).
You might want to think of your character as a role to play. I'm not even talking about using first person view here. I never wrote in first person view. Kind of grants you a nice overview if you write in third person knows-all-about-your-main-character view. That's like playing in third person view in a video game. You see what's behind you, but you're still playing your character.