A common problem is eating to the dose of insulin you are taking. Your dosing needs to be for for you actually eat. Dosing for your eating, not eating for your dosing. If you are on set doses, it can mean that you eat more to make sure you ate enough because of the dose you took...............hence too much food. Or you take a dose and start dropping too much at some point and then have to eat more. So if you aren't carb counting and adjusting your dose you need to learn to do that as soon as possible. I find prebolusing part before I eat and then taking the rest to what I actually ate works best. If you are carb counting make sure you aren't dosing too high and then eating more later because you are dropping too much. As that becomes just extra calories.
Another problem when you are "newer" to insulin. What you ate before you were diagnosed it was actually too much food for after diagnosis and starting insulin. Before you used insulin you probably were eating more because you weren't utilizing all the food you ate because of the lack of insulin. Hence it's common to lose weight before you are diagnosed. And then it's common to start gaining after starting insulin because it was too much food for you.
It is harder to lose weight as a diabetic, but it still comes down to needing less food/calories. And what one person can eat is just different from what another can eat. It's unfair, but it is what it is.
If you are on set doses you need to talk to your diabetic counselors before you make changes as too much insulin, or too little insulin causes problems. From passing out with a hypo to going into DKA from a lack of insulin and both can be very serious.