Did you have a particular word that gave you trouble?
I couldn’t say interesting. It came out in-ster-sting.
I clearly remember the day I finally said it correctly, running downstairs like I’d won the lottery yelling, “Mom, Mom, I said it, I said it!”. Everyone in the house knew exactly what word I was talking about. We were packing the car for a road trip. No telling how many times they had to listen to that word on that trip!
Not me, but my son. He had a toy take apart airplane. He just could not say propeller and it always came out as pimpiller.
Elucidate. I always pronounced it ee-loo-di-kate.
refrigerator
Ulysses
Ambulance. It was always amblyance.
I don’t remember any words I mispronounced more than through ignorance (my wife and I are amused that our youngest kids are like we were, reading far past their age and thus having huge vocabularies they can’t pronounce) but my oldest was probably in his early teens before he could get out “spaghetti” without effort. It was always “bisketti.”
I was an adult and used the company name “Rayethon” several times in a meeting before someone gently suggested it was Raytheon. At least no one laughed.
Cinnamon. It was usually something like “sim uh nin.”
I couldn’t do a wolf-whistle. Actually, I could hardly whistle at all. But I wanted to make that noise… so I said it. “Swisssle-sweeelll.” My family still teases me about it today.
My brother (or my sister?) had the same issue.
For me, it was venchtables. You know, broccoli, green beans, carrots…
Cripspspsps. (Crisps). Still trips me up now and then.
Cars went in a par cark.
As a big *Superman *fan as a kid, I had trouble pronouncing the word “invulnerable.” Problem was, I had read it many times in the comics, but never heard it spoken. So I routinely pronounced it “in-vul-NEER-uh-bul.”
That pinky purply colour- you know, ‘magneeta’.
Not just when I was a kid either, I still have to think about that one.
Luckily, my housemate also mispronounces it in exactly the same way, so we’ve just decided it’s an extremely localised accent 
An aside, really, but it took me forever to recognize that the popular storefront software is “Magento” and not “Magneto.”
Supposedly was “susposedly”
Spaghetti - “pisketti / pisgetti”
Pretzel / pencil- "prentsel "
Specific. Just couldn’t, was more like Spsisisfic
Can’t remember mine, but my daughter had the darndest time with “Burger”. It took her until she was nearly 4 before she could pronounce the first R. “I want a cheese booger.”
She speaks extremely clearly for an almost 5 year old, with a very large vocabulary and great diction, and has since she was 3, so it stuck out all the more.
As an aside, it’s quite humorous watching strangers listen to her speak in public, since she speaks like an 8 year old, but she’s positively TINY for her age, so she looks a year or so younger than she is…Many people think she’s 3, and then she starts talking in fully formed paragraphs using words most kids haven’t learned yet. It’s pretty funny.
It’s not that I couldn’t pronounce it correctly it was more that I was too stubborn to believe it was incorrect.
“Today we went to the libary.”
“You mean library.”
“What?”
“It’s pronounced library.”
“That sounds dumb. As I was saying, today we went to the libary.”
My son couldn’t “bemember” where he put things.
My niece called her toy frog a “shrog”. In her mind she was saying it right so if we ever called it a shrog she would get angry and say, “No, it’s not a shrog! It’s a shrog!”
My friend’s son couldn’t say breakfast. It came out as fuckfist. He was very entertaining in restaurant telling the waitress, “I want canpakes for fuckfist.”
My five year old pronounces “Elizabeth” as “Alibberdess”.
Um… my name. 
-Sincerely,
Nonna (Donna)