I will try to break down both grammar points and highlight the differences and similarities.
First of all, both are adverbs.
せっかく means that something is considered to have high value for the speaker, it is desirable. Something achieved through effort or rare chance/occasion.
Often used with だから, meaning that the speaker wants to make use of that occasion or のに (けど、が, etc) when the occasion was unfortunately wasted.
わざわざ means that someone puts effort to do something that takes a lot of time, labor when there is no need to do it, or there is some easier way. It can also express doing something on purpose. One goes out of his/her way to do something. bothers to do something.
Armed in this basic knowledge, let’s do some analysis:
You don’t know whether speaker subjectively considers it to be positive, or negative. It is objective fact.
(I) through great effort went to date (and consider it to be of great value)
(I) went on a date (I didn’t need to do it/ I didn’t want to) but still went
わざわざ is not used to describe something that happens regardless of that person will, the rare opportunity.
わざわざ doesn’t make sense in this case.
**On the other hand, せっかく cannot express doing something on purpose when there is no need to do that. **
Like angrily slamming doors.
The common usage of わざわざ is the appreciation of someone’s effort. It is very similar to English.
Despite being busy, he took all the trouble and came to my birthday party. (he put effort, although he didn’t have to go that far)
彼はとても優しい。 わざわざ, プレゼントを送ってくれたほど。
He is very nice. To the point of sending the gift. (put effort, even though he didn’t have to)
But わざわざ can also sound very rude:
I bothered to bake this bread, so eat it, please. (sounds self-important)
By the way, as you probably noticed, it is often used together with movement vocabulary.
It can be also used to express criticism.
Even though the supermarket is close, he bothers to go there by car.
In this case, せっかく is your choice:
I made this bread through great effort and consider it to be of high value, so please eat it.
Let’s do some sentence analysis:
I took great trouble to study Japanese for hours each day, but I got worried about whether I had passed the JLPT or not and got depressed.
せっかく nuance: I put a lot of effort to studying and consider it to be of great value (and I wanted to make use of it), but
わざわざ nuance: there was no need to study to that extent, but regardless I went out of my way and studied, but
Even though I went through the trouble of being transported to a parallel universe, instead of getting cool powers, I became a simple vending machine.
せっかく: even though there was a rare chance of moving to another world and I consider it to be of great value (and I wanted to make use of it), but
わざわざ: although I didn’t really want to move to another world, I went out of my way and did it, but
わざわざ cannot be used here since it expresses a long-awaited occasion/opportunity. Generally, せっかくの cannot be rephrased to わざわざの。わざわざの is a very limited expression.
Even though I went through great pains to build a house for my wife, we divorced. All of my efforts were in vain. I feel down.
せっかく: I built the house through great effort, and consider it of great value (and wanted to make use of it), but
わざわざ：although there was no need to build the house, I went out of my way and built it, but…
I hope the difference is clearer now