1) I think this statement - when you say "who have helped me throughout last year" - sounds perfectly fine! 2) The proper way to say it would be "all of last year AND all of December."

If December is in the same year as the "all last year" that you are talking about and you were only traveling in December then you would only say, "all of December". However, if you traveled all year including December, then you would just say, "all last year."

I don’t think the sentences look too bad. Saying “throughout last year” sounds a bit odd to me. I think keeping it simple and saying “all last year” is best. But that doesn’t mean “throughout”is per se wrong. You can say “all of” last year or just “all last year” and either one is fine. But, always go with the shortest form of saying anything. It’s considered better grammatically. The rule in English is say what you want to say using as few words as possible without losing the meaning you are trying to convey. That was what the famous and brilliant writer, philosopher, mathematician from the last century, Bertrand Russell, used to say. He even put this in a famous essay “How I write.” For some reason, English just hates wordiness.

Please, could you help me?

Please