Honesty is generally the best policy and I would show them if I were you with awareness the staff will be able to prevent you from taking that sharp knife sorry you are going through a hard time atm and I hope things will improve for you soon!

And if the diazepam is helping and you feel that you need it for now then just take it. You are in a crisis house, it makes sense that you are feeling worse than normal and so more likely to need the diazepam.