Learn Polish: Odkąd tu pracuję, moje samopoczucie jest bardzo dobre. - Since I've been working here, my well-being is very good.

In Polish, if an action started in the past and is still happening right now, you just use the present tense. The word Odkąd (since) already establishes that the timeframe started in the past, so pracuję simply shows that the working is still going on today. You don't need a complex "past-continuing-into-present" tense like in English.

What exactly does the word samopoczucie mean and how is it built?

Samopoczucie translates roughly to "well-being", "mood", or "state of mind". It's a compound word made of samo- (self) and poczucie (feeling or sense). Because it ends in -e, it is a neuter noun, which is why it takes the neuter adjective dobre (good).

Could I change the word order and say Odkąd pracuję tu... instead of Odkąd tu pracuję...?

You would be understood, but Odkąd tu pracuję sounds much more natural. In Polish, short, unstressed adverbs like tu (here) and short pronouns usually prefer to tuck in right before the verb rather than hanging at the very end of a clause.

If I wanted to talk about a job I already quit, how would this odkąd clause change?