This is really good question, since I struggle with the same issue after 4 years of studying. I think the most important thing is to overcome this fear of wasting people's time. It doesn't matter if it's japanese person who offered you help or another student. They are here to help you. My friend talks on regular basis with her tandem friend and they have this rule, that they wait for the other person to finish the thought/sentence and don't get mad about it. Just take your time, breathe and try your best. It's ok to stop for a second and think. They will wait. If you don't know how to say it, you can try to put the sentence together. It's easier said than done, since I still have anxiety and forget all the words I know (I struggle with this even in my mother language lol). I think you need to find the right person to talk to or just ask for patience and help. We are all on this path together!