How do I stop losing my train of thought in interviews?

You have stories ready, which means you have already done the hard part. The trouble is that under pressure they spill out tangled and you lose the thread halfway through. That is not a memory problem. It is a nerves problem, and nerves can be worked with. A story holds together when it has a simple shape. Think of three beats: what the situation was, what you did, and how it turned out. When you feel yourself drifting mid-answer, you can always return to that spine. It gives your mind a rail to hold instead of a blank field to wander. Practice each story out loud until the three beats are automatic, not word for word, just the shape. Say them to a friend or to a mirror. When the structure is solid, pressure can shake you without knocking you over. You are close. You just need a frame to keep the pieces in order.