Learn Polish: Kawa z cytryną była zbyt gorzka. - The coffee with lemon was too bitter.

Yes! za and zbyt both mean 'too' when expressing excess. You can say Kawa była za gorzka or Kawa była zbyt gorzka. zbyt sounds slightly more formal, but they are perfectly interchangeable here.

Why is the word for lemon spelled cytryną here instead of cytryna?

Because it comes after the preposition z meaning 'with'. This preposition requires the Instrumental case. For feminine nouns ending in -a like cytryna, the ending changes to -ą (an 'a' with a little tail), making it z cytryną.

Does gorzka change its ending if the noun is not feminine?

Yes. Polish adjectives must match the gender of the noun they describe. Because kawa (coffee) is feminine, the adjective is gorzka. If you were talking about a masculine noun, like a grapefruit (grejpfrut), you would use the masculine form gorzki.

Does zbyt have to go exactly where it is in this sentence?

Yes, zbyt usually needs to stay right next to the word it is modifying. Since it is emphasizing how bitter the coffee was, zbyt must go immediately before the adjective gorzka.