Nominative to instrumental?

As an exercise I was given a series of sentences to rewrite changing the structure and grammar from nominative to instrumental:
'Jan pracuje jako nauczyciel' (Jan works as a teacher).
'Jan jest nauczycielem' (Jan is a teacher).

Masc/Fem distinctions also appear.
Lekarka/lekarką. (f)
Inżynier/inżynierem. (m)
Policjant/policjantem. (m)

Here are examples of which I am unsure:
Beata pracuje jako sprzedawczyni.
?Beata jest sprzedawczyniką?

Maria pracuje jako psycholog.
Maria jest psychologką

Iwona pracuje jako księgowa.
Iwona jest księgowaką

I'm fairly sure the above 3 are wrong.
If someone could correct them and explain the reason/rule (or exception to rule) it would be a great help to mr.

Also Uczeń becomes Uczniem (masc nominative to instrumental)
Does the female nominative Uczennica become uczenniką ?

sprzedawczynią
psychologiem *
księgową
uczennicą

So generally, masculine nouns in instrumental case get the ending -em/-iem, while for the female nouns the final "-a" changes to "-ą" (probably there are exceptions, but I don't have the time to think about it right now).
You are trying to add -k to those forms, but you got the wrong impression, form the "lekarką" example.
-k ending has nothing to do with the instrumental case.

(Nominative) lekarz - lekarka (here the masculine -rz ending becomes -rka in the female variant, so this ending (-ka) serves often to make a female version of a male noun (especially words borrowed from the Latin), but many words (especially those with Slavic roots) that have existed in a female form for long time have different endings.

psycholog/psychologiem * - identical (masculine) forms for both male and female!
That's the patriarchic tradition, where many jobs were reserved to men only, now, with the social changes of the XX and XXI centuries, the language is simply "too slow" it doesn't change quick enough to describe the new reality (women excersing the previously "men-only" professions), so some words are still existing only in the male forms, some have had official female variants (like lekarka) for quite a long time, some got the female variants recently (filozof - filozofka), some are still considered (by my ortographic dictionary) as colloquial (for example psycholożka) or incorrect. As you may see "-ka" ending is popular here for the female versions.
You had male form used in nominative, so it has to be psychologiem, but if the nominative sentence was
"Maria pracuje jako psycholożka", then the instrumental would be "Maria jest psycholożką".
(You can see the same in English, actor-actress, waiter-waitress, because those professions were allowed or even more popular among women than among men for long time. Then you have some recent addition - policeman>>policewoman, and chairman>>chairperson, but some professions have still only the male versions, if I'm not wrong: doctor, psychiatrist etc.)

That's a real curve ball I was thrown!
I was aware of the principle of 'Pani Dyrektor' (A female title and a masculine gender profession) but I was given no hint that the exercise contained anything other than varients of the same process.
Thanks,
Krzysztof.

Are you taking a course or are you self-teaching through a certain book?

I'm taking one-to-one lessons (internet skype connection).
Before the lesson I receive an e-mail with attachments of pdf/audio files. The tutor and I work through the material and I listen to the audio files to aid my 'homework'.
As well as this method I use some self teaching materials (books, websites) and find the input of others invaluable.
Especially the feedback and direction I am given by the members of this forum.