Grammar usage

I am happy with my job so far is correct.   I am happy with my job until now is not correct but if you change the verb to past tense, it becomes grammatical: I was happy with my job until now.

The two sentences have slight, but important, differences in meaning:

I am happy with my job so far means that I have been happy with my job ever since I started it, and that happiness has continued right up to the present moment of saying/writing this statement.  I've been happy with my job so far would have the same meaning.  Adding the adverbial so far to the statement, however, implies some doubt, or at least uncertainty, about how much longer my happiness will continue in the future.  So far is also only used at an early stage of a job, etc.; if you have been working at the same job for five years, for example, so far would sound strange.

I was happy with my job until now means that I was happy with my job until something changed very recently, or changed just now, and so I am no longer happy with it.  I'd been happy with my job until [x happened]  would convey the same meaning; you couldn't use the Past Perfect with now, however.

I hope this helps.