Ikea, so sorry you (and Nana
) are struggling 
How can you keep yourself safe? Have a plan, think about what you might do if you feel you want to hurt yourself - tell your DH? phone the Samaritans? do you have a contact no with somebody on the crisis team? is there a trusted friend you'd speak to? does distraction help with intrusive thoughts? does playing music/eating something nice/going for a walk help?
These are just ideas and ignore anything that is totally inappropriate - the point is make a plan in advance on what safety net to use if you need to. Often knowing that there is a plan helps people feel better.
Nobody can help you or even try to help you if you are not honest. Please be honest, otherwise why engage??
You WILL NOT always feel like this - if you do something you cannot take back, you rob yourself of the chance of ever feeling better, making new experiences or understanding more about your illness. Suicide is a dead end street and only SEEMS like a way out. It isn't.
Working with bereaved families as I do from time to time, please know that there is no loss as devastating as losing a loved one to suicide (with the possible exception of childhood death) and those left behind always forever feel guilty and in need of answers which do not exist. Please trust me, should you ever feel that your DH or family and friends would be 'better off without you', that is the bastard illness talking and NOT true. It never is.
Keep engaging with the help there is.
NB The antihistamine you were prescribed is often used as a mild nighttime sedative.