MRIs suck. I'm claustrophobic, but no one had warned me about the cage, and I was terrified. I cried a little bit in the machine, and also had no earphones or earplugs. Not fun, and getting help to cope with it is sensible, nothing to feel silly about.
Please remember - you have survived one before, and you will survive this one!
try to spend the next few days focusing just on your breathing, so that during the MRI you have had practice. And remember that it will end. It will! 

I'm guessing they have given you 2mg, if you have not been prescribed it before? Expect to feel it after 20-30 minutes, it will last a good few hours, 4-6 seems to be average. I would also recommend taking it an hour before the scan, because it will allow you to get used to the feeling, and to do your focused breathing.
Tell the medical team that you have taken the diazepam, and perhaps write a quick note before you reach the hospital, stating simply that you had a panic attack previously - the team should know, but this will guarantee it, and means that you don't have to talk about it (because I know that can make my panicking worse!)
All the best for the scan, and I hope the results bring good news.