Big spoonful of nut butter in the porridge will add protein and fat. Also rather yummy. You could also make the porridge from half oats half oat bran. And add some chia seeds or linseeds as sprinkles, and natural yoghourt to cool it down. Then a couple of fried eggs on toast. Pretty much one way I breakfast (a fruit and veg smoothie often also) and my weight is relatively stable. But I generally eat frugally at lunch (yesterday was smoked mackerel and salad, no bread) and a bit more for supper (cold duck with fried potatoes and some pak choi with soy and teriyaki sauce), snacks were one slice (not too big) of cake with tea, a handful of nuts or so, a small orange and half an apple (we are using up windfalls from the garden, so that was a whole apple if you see what I mean).
Which leads me to the main point, I think, which is portion size (alluded to above). One person’s slice of cake, portion of chips whatever can be very different from another’s. You could just try doing a bit more of what you do.