In theyour last mail, you told me you were thinking about becoming vegetarian.

If handwritten, maybe "mail" --> "letter".

The fFirst, you have to keep in mind is that it can be a hard transition, especially if your current diet is highly based on animal products or if you usually go out to eat.

A way to make the change smoother, could be to think about the very goodgood that you're doing to the animals and the environment with your new lifestyle.

Although, the way I see it, the most important overpart of the change is your health.

Eating more vegetables and fruits wouldfruits and vegetables will really improve it,; you'll gettake in more nutrients and improvinge your immune system.

It is only a minor thing: in my brain, "fruits and vegetables" sounds more native/natural. :)

But, on the other hand, you'll have to check your B12 vitamin B12 levels regularly due to the fact that this vitamin isn't present in vegetarian food.

Due to the fact, that this vitamin isn't present in vegetarian food.

Moved this clause to the sentence before; it is incomplete

Let me know if this information helps toyou make up your mind.

Best regards.wishes,

This seems like a casual letter, right? They are your friend. "Best regards" sounds like a business email. You could also say "From, [yourname]"

In the last mailing you told me you were thinking about becoming vegetarian. OR¶
In your last letter you told me you were thinking about becoming vegetarian.

'last mail' is too general a term, be more specific with either 'last mailing' OR 'your last letter'

A way to make the change smoother, could be to think about the very good you're doing tofor the animals and the environment with your new lifestyle.

there are a few filler words missing.

Although, tThe way I see it, the most important over the change is to your overall health.

Although is not necessay here, the word order needs to be adjusted.

Eating more vegetables and fruits would really improve ityour health, you'll get more nutrients improving your immune system.

What your improving is unclear when the word 'it' is used, replace 'it' with 'your health'.

DThis is due to the fact, that this vitamin isn't present in vegetarian food.

Let me know if this information helps toed you make up your mind.

Sounds awkward