The boots dont actually sound like they are too big to me, it just sounds like the heel pocket is not working for you. It may be that the liner has packed out, the heel pocket may not work for your heel shape, the instep of your foot may be too low for the shell, your foot may be pronating in the boot or your Achilles may be tight. All these things can be sorted out to varying degrees by a good bootfitter.
People hear are too quick to say a boot is too big. Boots take skill to fit correctly and unfortunately there are a lot of bad fitters out there. But if you find a good one you will be amazed what can be done.
What I would do is take your current boots to a good fitter. Have them see what they think of the fit. If they are good they will tell you if they think they can be refitted and not just try to sell you a new boot. If they dont think they are a good fit then at least you know and you can start again.