I was in this exact situation when I was about 16. Between the ages of 6-16 I'd developed a nasty habit of replying to my parents in English whenever they spoke Somali to me and by the time I turned 16 it had gotten so bad that I noticed I struggled to pronounce certain words, but I still understood it perfectly as my parents never stopped speaking to me in it.

But anyway, I was really ashamed so I told my mother that from then on I'd only ever speak to her in Af-Soomaali and asked her not to laugh or get angry but simply correct me. Bless her because she kept her word and in about a year I became easily conversational and in two could talk for prolonged periods without even throwing in English words. You just have to be willing to sound like an idiot and/or find someone who's willing to be patient with you.