I've gotten used to eating it since my boyfriend and I started to liveliving together.

Since it's an ongoing action, the -ing forms of the verb will be better here.

Before this I had aI used to drink tea and eat some sweet things like cookies for breakfast.

Is this referring to before you started living together? If so, you might say "Before we started living together, I used to..." or "When I lived alone, I used to..." to clarify the timing.

We haved been eating cereal in the morning for approximately one year year or so.

For an informal work, "a year or so" sounds more appropriate than "approximately one year", which carries a formal feel.

ThenAfter that, we switched to buckwheat porridge.

We add some fruits and other toppings to the dish so we've noour porridge for variety so we won't goet tired of it.eating it every day.

Just a little more explanation needed to make it sound more natural.

Sometimes we will cook something else, for example, like an omelette or some pancakes.

But we almost always end up cooking the porridge more often.

This phrasing gives the idea that, while you like other things, you know that the porridge is the constant part of the meal.

There's one more thing I usually eat, it's chocolate.I also love to eat chocolate in the morning.

I think it's a bad day if I don't get anmy chocolate., it's going to be a bad day.

More natural phrasing for both these last sentences.

Fun fact, I have porridge for breakfast every day.

I've gotten used to it since my boyfriend and I started to live together.

Before this I had a tea and some sweet things like cookies for breakfast.

We add some fruits and toppings to the dish so we've no don't goet tired of it.

Sometimes we cook something else,; for example, an omelette or pancakes.

But we cook the porridge more often.

There's one more thing I usually eat, - it's chocolate.

Or, to write without a sentence inversion, you could say "Another thing I eat is chocolate."