Why, exactly, is it so difficult for English speakers to pronounce the ð and Þ (voiced and unvoiced dental fricative) sounds?

I speak English as a native language, and Icelandic as a second language, and I have spent so much time trying to get those “th” sounds right, and I still can’t. The only way I can get it right is by putting the tip of my tongue between my teeth, like a kid with a lisp, and then speaking without blocking the airflow around my tongue. It’s so hard though, and it gets exhausting after a while. I’ve always thought I’d be good at languages but I just cannot pronounce Icelandic properly no matter how hard I try. <br><br>I know it’s not a problem unique to English speakers, but I’m an English speaker, and it’s the same problem that I see a lot of other English speakers asking about.