How dificult is it to get into a PhD program years after quitting another?
It will be very difficult indeed. 

Having a second-class undergraduate degree I was unable to get any PhD position and was advised to take a Masters course to improve my grade. If you fail the PhD or abandon it then you will find it almost impossible to get a funded place again - you would need to fund yourself, and to demonstrate successful completion of whatever you did in between. You might be successful if you had to withdraw early on for health reasons, but not as late as the 5th year. I presume that you are already past the usual finishing date, which in itself is a negative indication.

You will do far better to apply for an extension or a suspension of studies. You will have to re-register every year but this could give you the time you need to reassess what has gone wrong and how you can steer your PhD to a successful conclusion.

The starting point is to have a thorough discussion with your supervisor and the tutor in charge of the PhD programme (assuming they are not the same person), to discuss the options. If at all possible avoid taking a major break because it can be a huge obstacle to get back into it, and the field will move on while you are away so that your research becomes less relevant and harder to "sell". 

In the end you just have to grit your teeth and get the work done, regardless of how dissatisfied you are with the outcome. Lack of a PhD may not make a difference in your current job but it will make a difference to your career progression, and will probably close off options of moving into an academic career later on.

The fact that you have a job might be a distraction which is keeping you from completing your PhD. It might be better to ask your boss for 3-6 month of unpaid leave to complete your PhD, or at least get it back on track.