I have a question about sentence sturucture here

Question

I'm writing something. Here are sentences that I think are all suitable. Can I use whichever I want?

One of the happiest moments I’ve had was when I was doing something enthusiastically, like a contest, a relationship with a woman, or studying English.

Whatever it is, the happiest moment is when I find myself enthusiastically doing something...

1. When I’m doing something enthusiastically, it is the only moment when I can feel alive.

2. I can only feel alive when I'm doing something enthusiastically.

3. It is when I'm doing something enthusiastically that I can only feel alive.

Answer 1

Right now, only 2 is correct. However, 1 and 3 could be correct with a few minor modifications.

When I’m doing something enthusiastically, it is the only moment when I can feel alive.

This sentence sounds awkward because of two things. First, your usage of the word "it" is wrong. Here is the right way to use "it". Let's say I have a car, and I want to tell you that it is red and fast. So I say:

I have a car that is red. It is really fast.

In the first sentence, I don't say "I have a car that it is red" because I am directly referring to the car in the same sentence. The "it" is unnecessary.

I do say "it" in the second sentence. The second sentence really means "The car that I just mentioned to you is really fast." However, it is a lot easier to just say "it" and you understand this to mean my car from the context. Actually, the definition of "it" is

used to represent an inanimate thing understood, previously mentioned, about to be mentioned, or present in the immediate context

This sentence also sounds awkward because you use "when" twice. I would drop the second when and make your sentence "When I'm doing something enthusiastically is the only time that I can feel alive."

It is when I'm doing something enthusiastically that I can only feel alive.

This is correct, but it's not what you're trying to say. This means;

There is a time that the only thing I can feel is the feeling of being alive. This time is when I am doing something enthusiastically.

I assume you are trying to say:

If I am not doing something enthusiastically, I do not feel alive. If I am doing something enthusiastically, I do feel alive.

To fix this, move the only to before "when"

It is only when I'm doing something enthusiastically that I feel alive.

Answer 2

There seems to be an inconsistency in the sentence you provided as context. You state "one of the happiest moments," which I take to mean you're focusing on an individual moment out of many, yet you list multiple moments in the same sentence as well as imply that you're talking about all of the moments in which you were doing something enthusiastically.

I'll assume that you intended to describe all of the moments. In that case, I would make the following corrections.

Some of the happiest moments I’ve had were when I was doing something enthusiastically, like a contest, a relationship with a woman, or studying English.

Whatever they were, the happiest moments are when I find myself enthusiastically doing something...

As for your question, the second sentence is the only one that sounds natural and is completely error-free.

The first one doesn't make sense because the last clause of the sentence implies exclusivity that the first clause does not (same issue with the context sentences).

The third sentence is incorrect as it indicates exclusivity in the wrong part of the sentence, but it can be fixed by simply moving the word "only."

It is only when I'm doing something enthusiastically that I can feel alive.