What would be the karmic result of worrying? How come worrying is a bad karma ? Buddha stated that Having a confident mind gives a good rebirth ( in heaven etc.).

Most of the time, I worried about not able to complete a task within the desired time.

What would be the karmic effect of this worrying and any other worries ?
Worrying, because of its obsession with self (i.e., "I won't finish") is a form or wrong mindfulness. Right mindfulness attends to what will help oneself and others (e.g., "What effort and action would be of value now?"). 

But you ask about the effects of worrying. These are listed starting here:


  AN10.114:9.1: Wrong mindfulness is a bad principle.  Right mindfulness is a good principle. And the many bad, unskillful qualities produced by wrong mindfulness are bad results.


The remedy for worrying is to become aware of and be mindful of those around us. In this way we can communicate and coordinate, building what is needed by shared effort rather than by fixating and worrying about "my job" or "my things". Worry is often tied up in Identity View. And the escape is not laziness or blissing out. The escape is right view, right thought, right speech, right action, right livelihood, right effort, right mindfulness and right immersion.