Some speech sounds are harder to master than others. I've wrestled with Spanish for years and I've never been able to do "rr". Linguists call this an "alveolar trill". It's also in Italian, Russian, and a bunch of other languages. The single "r" is fine but "rr" always gives me problems. Any time I try to do one I get some other sound, more like the "ll" from Welsh. Fortunately, Spanish speakers can understand.
NOT! Do you know how you hear lisps? That's how they hear it. Either they ignore you or they make fun of you. They don't help you. It's a common speech impediment. I don't know if it's genetic or if I'm not used to it. I can make lots of other sounds easily. The French/German "r", a "uvular trill", is real easy for me. I use it when I "speak" Spanish because it sort of sounds like the Spanish "rr". I want to learn the "right" sound someday. Most instructions for learning it make no sense. I may have to accept that I have a Spanish lisp. But I don't know anyone who learned English as a second language who talks like a native. No matter how fluent they are, they sound foreign.
It's ridiculous that I can do the Dutch "g", Welsh "ll", Italian "gli", etc. but not a sound that's common in lots of languages. I don't find any German sounds hard, including their R, their umlaut vowels, or their "ch" sounds. There are other sounds I don't know how to do but they're not in languages I encounter every day.
I read somewhere that most people can do the Spanish/Italian one or the French/German one easily, but not both. I also found out there's a disorder called Ankyloglossia where the tongue is tied to the floor of the mouth. I may have this. I can barely stick my tongue out.
The "th" sounds in English are very hard to learn. (1, 2) They were the last sounds I learned. A lot of foreign speakers use "s" or "f". Some dialects use "s", "f", or "t" for "th". Thankfully, they're rare in other languages.
Linguists call the English "r" an "alveolar approximant". It's only in a few other languages. Here is a list of R sounds in different languages.