Welcome to the FDMB! Bella is a gorgeous girl!! Great job getting your signature up and the spreadsheet going too!
Vetsulin is unfortunately, not a very good insulin for most cats and from the data you have already, you can see why. It tends to hit hard and fast and then wear off before the next shot is due. Also, if she's been on it for over 2 years and isn't better controlled than that, it's time to try a different type of insulin. Vetsulin is called Caninsulin in the rest of the world (which is a good hint to which species it works best in...Canine's!)
Lantus, Basaglar and Levemir are all human insulin's that work very well in cats. They're expensive in the US, but most of us buy ours from Canada where it's 1/3 to 1/2 the price. Here is some
information on Buying insulin from Canada
ProZinc is a veterinary insulin that works better in cats too. It's available at Chewy for about $110 but although the human insulin's may be more expensive up front, they also last longer and end up costing about half as much per unit.
The
2018 AAHA Guidelines on diabetes management recommends Glargine (Lantus/Basaglar) and ProZinc for cats. Maybe if you provide your vet with them, they'll agree to let you try something else.
I've thrown a lot of information at you! Feel free to ask any questions you may have!