Learn Polish: Trudno mi mówić o tej emigracji. - It's hard for me to talk about this emigration.

With impersonal words like trudno (it is hard), warto (it is worth it), or szkoda (it is a pity), the person experiencing the feeling is put in the dative case. So instead of using dla mnie (for me), Polish just says mi (to me).

Could I use the long form mnie instead of mi in this sentence?

Generally, you should stick to the short form mi here. The long form mnie is only used for emphasis, for example if you wanted to point out it is hard for ME specifically, but maybe not for someone else. Since this is a standard statement, the short, unstressed mi is the natural choice.

Why is it trudno and not the adjective trudne?

Because there is no specific noun acting as the subject here. When you make a general statement about an action using an infinitive verb like mówić, Polish uses the adverb form ending in -o. You would only use the adjective trudne if you were describing a specific neuter noun, like To jest trudne słowo (This is a hard word).

Why does ta emigracja change to tej emigracji at the end of the sentence?