These are mixed up all the time by students. Maybe it's because わざわざ is just so fun to say?
せっかく shows something that is done through great trouble or time to reach a certain point or condition. It is an adverbial noun, so it is possible for it to act as a noun. Again, there is some sort of big cost implied, and it is that cost being put into good use that's being insisted on.
There are several grammatical environments that せっかく can be used in.
These are simply applications of せっかく with other patterns. Of course, you should always be aware of how polite you should be to someone.
Since I've taken the trouble to study Japanese, I want to become able to speak Japanese fluently.
Nuance Note: んだから implies that the person you're speaking to is well aware of your Japanese skills.
All my troubles have been done for nothing.
All my pains were in vain.
For your great kindness, I accept.
Please try not to forget Japanese, which you've taken the trouble to learn.
Even if you make the efforts to go to Japan, if you don't speak with Japanese people, your Japanese won't get better.
Since you've gone through the trouble to come to America, please stay for a while.
Although I spent so much energy writing my essay, it got completely erased!
9. せっかくやれるなら、やってほしいんだが。 (失礼な言い方）
If you could do it, I'd like you to, but...
Orthography Note: せっかく can be written in 漢字 as 折角 or 切角
せっかく VS わざわざ
せっかく gives a feeling that something should not be wasted because one has put a lot of effort into it. It can be used as a nominal, and it is often used in refusing a request. わざわざ, on the other hand, can only be used as an adverb and describes that there was no necessity to go through the trouble. It is often used out of care for a person.
I'm sorry for having you come all the way while you're busy.
I went all the way to Kitami to eat yakiniku.
Thank you, but don't trouble.
Since I went through the trouble to make it, please try to eat it.
It's alright for you to not go all the way home to get it.
Although it's a long-awaited holiday, I have to work.
Thank you very much for coming for me although you are quite busy.