You mentioned you find heavy cream nasty. Drinking it straight isn't for everyone, nor is it very effective for gaining. Instead, mixing it with something that has more substance can help it digest.
Personally, heavy cream, 1 pint, and one box of instant pudding, usually vanilla or banana cream flavour. If you're used to stuffings, then this will definitely help. We calculated this out to almost 2,000 calories per batch, and use this to pad calories in tight times.
You can mix this without a blender, and its better than most gain shake ideas we have had. You can add nuts, for even more, but that requires a blender and doesn't taste great with vanilla. Other than that, our diets sound pretty similar... Kita will note more micronutrients, so eating things like veggies and mushrooms, is actually *really* important to gaining.
Macronutrients are the materials, but micronutrients are the catalysts. So you might have overcorrected into eating too many macronutrients.
Physical activity can make a difference, but not usually through burning the calories. It is through endorphin and neurochemistry stuff; or, if your job has you doing things like lifting, squatting, etc, then muscle mass competes with your fat for these resources. Not having a physical job did a lot for our gains once the muscle mass reduced.
Lastly, hormones are a big factor. Some people can only get so big, and sometimes you'll plateau for a good while before your hormones shift and you break that barrier.
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