For those saying why are you surprised no response to diazepam when it is for stress for you and you weren't stressed when you took it.
Whilst agree it isn't the best indicator of how it might affect stress levels, it is still medication with many effect.
Sleepiness, feeling dosy, sometimes feeling a bit down the next day, all these things will happen regardless of whether the person has stress at the time, sometimes but not that time, or is a buddhist monk.
So - if you didn't feel any of these things OP, maybe you just have a high tolerance. I agree you may find that in a stressful situation it may actually have an effect on calming nervous symptoms, not possible to know until you are actually in that situation, but given that you felt nothing on an amount everyone is agreeing is a large dose, maybe you have a higher tolerance for these types of medications. Everyone is different, either through experience or just natural body chemistry.
Plus as has been mentioned, an amount that almost knocks someone out can not touch a severe anxiety panic in another person, and this is related to the level of anxiety. For example, if you have a headache you may take ibuprofen and the headache eases. If however you break your leg and have surgery, the same dose of the same medication is probably not going to make any difference!
Given the nature of benzodiazepines, it's better not to up the dose too far (above 10mg is getting into serious territory) if you have no success with them due to the quick addiction, but to switch to another medication. Another benzodiazepine may be an option, diazepam is the gentlest in terms of effect and addiction, but they are still serious considerations. Others, lorazepam for example, very strong but serious withdrawals and memory gaps, so better for situations where very serious things are going on.
Again it depend on the type of stressful event - whether you need to be alert or whether being sleepy and a little bit out of it would actually make things better. It's fine to sleep on a plane for example, it's not so good if you find yourself falling asleep at your wedding.
Beta blockers I took once when I was about 21, a foolish doctor gave me them when I said I had a lot of pressure at university with work to do and to get the grades I wanted. I didn't say I couldn't cope with it or it was a problem, I could and it's normal, but he was terrible. He said take these, I hadn't yet learned to say no way you are wrong and find someone else to work with, so I took one. I couldn't walk properly and fell whoozy. Misprescribed. So they are not always the answer either.
There are lots of options, I would really suggest going to another doctor and discussing with them.