This doesn't necessarily sound like a stutter to me, but it does sound like something that needs to be checked out if possible. When I stutter, I physically can't say the word; it's like my throat and chest lock up. You don't mention a physical struggle with certain words, rather that you can't find the next word to say -- that sounds like it could be something neurological, maybe something like aphasia syndrome. Can you talk to your parents and ask to see a speech therapist? Either way, that could be really helpful for you and help you find answers; unfortunately none of us can diagnose you over the internet. Hugs, this sounds so frustrating for you.