There are a number of medications used for people with anxiety issues that have also been used with pets. These include newer antidepressant meds like fluoxetine (Prozac), sertraline (Zoloft), and paroxetine (Paxil), which are known to be effective in people for both depression and anxiety, as well as older types of antidepressants like amitriptyline (Elavil) which has a sedative side effect but in tiny doses can help with calming. These meds take 2 to 4 weeks to reach maximum effectiveness in people, not sure if the same would be true with a cat.
You mentioned Valium - that is an antianxiety medication, one of a class of meds called benzodiazepines. Others in the same class would include Xanax, Ativan, etc. In people these medications are effective in managing chronic anxiety as well as anxiety that is tied to specific events (such as going on an airplane), and to help bring a panic attack under control. In my professional opinion (I'm not licensed to prescribe, but I am a health care provider#, with people, these meds are best used in the latter two ways, rather than for chronic anxiety due to addiction potential. I have no idea if a cat would become addicted, meaning develop a tolerance and need larger doses over time, but a benzo might help with particular situational anxiety like a visit to the vet, travel, new or extra people in the house, etc., as a supplement to the calming products for daily stressors. Another antianxiety medication that takes time to reach effectiveness and is not addictive is buspirone #Buspar). I don't know that it has been used with animals other than people, but someone else mentioned it in another Q&A, so maybe it has. Anyway, it has some nasty side effects, and of the people I've known who took it, most stopped duet to side effects, even if it was working. The primary side effect is one that I've heard described as feeling like one has been "hit with a Mack truck" 20 - 30 minutes after taking the daily dose, and having to either wait it out or struggle through it for another 15-30 minutes before this effect leaves. Not sure how this might effect a cat or how she would handle it.
All of these meds require prescriptions, so I would suggest talking to your vet. He or she may be aware of other options, as well as knowing more about dosing, side effects, etc., of using any of these meds with a cat.
Good luck with her.