There might be many contributing factors to why one fails units at university. I'll just mention the obvious ones:
Motivation
Not enough time from work
Its just too hard!
Basically, if motivation is a factor, you need to really ask yourself if this is what you want to do for the next 3,4,5 years... if the answer is no, ask yourself what you would love doing for the next 3,4,5 years, then transfer to that course.
On the other hand, if you like what you are doing but are too lazy then you have two options; stay and put in your all or drop out..you cant be half arsed at uni. If you need help getting motivated, dont just think about the uni work, but watch a few movies you like, listen to some music you enjoy in your spare time that might motivate you, this helps me a lot.There are a lot of movies a person can watch that can change the way they think about life and motivation in general. I recommend watching something like Fight Club or Gattaca...I know this might sound silly, but people can get motivated overnight..you just need that spark. Fuck tacking life half arsed, spear tackle the cunt.
If you're not getting enough time from work, you need to make a choice between the two options. Part time/casual during uni is ok , but if youre in job that requires lots of hours per week then you need to make a choice; the job or the uni. You cant have both unfortunately.
If you think its too hard for you, then I think you should drop out of uni in my opinion. Nothing is too hard.