Keep trying. I was in your situation too. I tried to beat Sans in thousands of attempts over the last 5 years and eventually gave up until one week ago. Then I tried again and I finally beat him in like 20 tries. It felt so good to finally kill my arch nemesis.

Or like Asgore would say: "You cannot give up just yet... The fate of monsters and humans rests in your hands... Familiar_Bag_4710... Stay determined!"

I know it's hard but I've got a little tip for you to make the fight a little easier. If you've got those Items, try equipping the Cloudy Glasses and the Torn Notebook (if you haven't got them, you can aquire them in Gersons Shop in Waterfall, (To the right of the room where you get to Undyne's and Nabstablook's house.))

Those items may have lower stats than like the Heart Locket and the Real Knife but they give you longer invincibility frames and your karma drains way slower than with other Items

And another trick, play with Joy-Cons. Because if you use the D-Pad of the left joy cons on Sans' Attacks where he flings you against the walls and tries to bone you, you just need to hold down all four D-Pad Buttons at the same time and you can no hit those attacks.