I totally understand your frustration. My boys were on NPH(Novolin), one for ~5 yrs the other for a month. NPH is very hard hitting insulin. It's really not a good choice for cats. It would drop my boys #s by almost 1/2 w/i the first 2 hours, maintain "ok" #s for a couple more hours and then back up they went. It was ridiculous! Once we started Lantus I got 1 boy into remission(he was newly diagnosed) and the other was well regulated. You will be thrilled once you see the change start. But, it does take time to see results. the depot has to build which can take ~ a week. So, you generally don't see immediate results. But patience is key.
I'm going to ask that you make a notation on you SS when you began Lantus. Check out other peeps SSs how they indicated the switch. I made it very obvious on my SS.
So, it looks like you vet told you to start with the same dose of 5 units? If so, that's the first thing that needs to be changed. That is too much insulin. With NPH those doses may be needed but, generally not with Lantus/Levemir.