There is a limit to what diet and other medications can do. Ultimately, if you don't have sufficient insulin then you need to replace that with injected insulin. Your levels at the moment are as you know very high.
You say that you put on weight with insulin but had you actually lost it before you started taking it? If you don't have enough insulin then you do lose weight (but have high glucose levels) , you are 'weeing' calories out. When you start taking insulin, your body can make use of the glucose and so you can gain weight.
The mixed insulin you were using isn't very flexible. There are other insulin regimes. Perhaps you could talk to the doctor about using what is called basal/bolus or MDI. This means one (sometimes 2) long term injections to cover glucose released by your liver between meals , then a separate 'fast' insulin to cope with each meal/
As you can vary the insulin for each meal, then you can vary the amount you eat. This means it is easier to control calories and weight.