What's the difference among those sentences?
Since then, I have been on a quest for a better livelihood. I have been
searching and searching for a good job- for something I love to do.Since then, I am on a quest for a better livelihood. I am
searching and searching for a good job- for something I love to do.Since then, I had been on a quest for a better livelihood. I had been
searching and searching for a good job- for something I love to do.Since then, I was on a quest for a better livelihood. I was
searching and searching for a good job- for something I love to do.
Best Answer
The present perfect tense in the first example is correct. Since a particular time in the past you have been (and still are) on a quest.
The second present continuous example makes for uncomfortable reading. The present tense is not suitable in a construction that begins with a reference to an earlier time. A native English speaker would be unlikely to say this.
The third past perfect example would be allowable if you continued to say that you had been searching until something had intervened to end your search. "I had been searching until I found what I was looking for".
The fourth past continuous example is not correct. It would be fine to begin this sentence with "last month" or "in 2015" but it can't follow "since".