Hi Norah, I see you posted in April and perhaps you have gotten a great job by now! I hope so.
There have been some good suggestions here. As others have said, almost all interviewers ask similar questions, so have a basic answer for each (tell me about a problem you solved? about a success? etc.) Here are a couple of things to keep in mind: silence is OK, pauses are OK, and you don’t need to give five-minute answers. Just as important are making eye contact, smiling and not overly fidgeting.
Many people cannot think “in the moment”, so it’s ok to pause for a second or two.
Norah, can I ask: is your perception of being unable to organize your thoughts limited to interviews, or has it also affected you in other jobs or at school? Some people become very nervous at silence and seek to fill any gap of silence with a cough, giggle, or “rambling”. Some become nervous if they feel that attention is focused on them, such as when an interviewer or a co-worker is looking at you waiting for an answer, or you’re a meeting and all eyes are on you.
If any of this rings true, and if your circumstances allow, I might suggest trying some sessions with a therapist to learn and practice some strategies (and it does, absolutely, take practice and constant reminders to yourself, but I’m here to say that change will eventually happen). But, apart from that, learn to project confidence in other ways. Sit up straight, but be relaxed. Smile. Smile (inwardly, outwardly). Give yourself a pep talk on your way to the interview (“I’m perfect for this job.” “I’ve got this.” List positive points about yourself.)