Do you generally use that much data in a month? Or have you just been using a little more than usual the last couple of months? You do have full speed 4G LTE so some sites will use more data than throttled 4G LTE. Any sites with a lot of video such as YouTube, Snapchat, Instagram etc....can really suck your data dry especially if they are left on in the background.
You need to do some maintenance on your phone and monitor your data more closely and you will be able to keep within plan allotment. This is important as since you have discovered your plan was eliminated well over a year ago as was any opportunity to have full speed 4G LTE. This may be important to you to keep so changing your plan now means you will lose it and the full speed data forever.
I suggest you do the following and test out on your next 30 day cycle if it helps you stay within plan allotment. Set your data tracker and update the dates when a month has more or less than 30 days in it ( like October.) Set your data warning and data limiter. I have mine set at 25% ( warning) and 50% (limiter). This way when you've used 1.5gb you'll be warned....you can note to yourself.....are you one week into your plan? Or one day?!! Again at 50% (3gb) your data will shut off. Are you two weeks in? This also a chance to review your apps for data usage and see what's using most of your data and limit your use of the ones that are high users....and try to use them when you're on wifi only. Then reset these again remember that you now have 3gb left and set the warning and limiter accordingly.
If you have WiFi assist or the android equivalent enabled turn it off. You don't need to be using mobile data accidently when you have access to wifi. Go thru your apps and turn off all background data usage this is just a waste of data and you won't notice any real difference in your apps performance (and you'll see how much data you're really wasting in background data usage.) Stay on top of this kind of data tracking and occasionally compare your phone's data tracking to the data counter in your account and you'll find it won't be off by much. You should be able to stay well within your data means...and keep your plan with full speed 4G LTE.
