The first year is always the hardest, getting used to balanceing everything, the fact that unlike high school, no one is going to chase you for assignments etc. I think 3/8 subjects that year i got 50s, another 4 were between 50 and 60. But then i pulled my finger out over the next 3 years, chalked up some decent marks and managed to scrape into a phd. If you had told me that was possible in the year and even some of second year, i would have punched you in the face for talking bullshit.
As for the part time work, i would definately go for that, experience is almost as valuable as the degree, but make sure you are focused on the theory not the job for the next couple of years