Just think of this as a very, very lightweight version of real-life priority-setting. Figure out the relevant commitments of time and effort; don't forget to factor in a reasonable amount of alone-time; then just put yr damn foot down, and say that these are yr needs.
If you can articulate yr needs, without being a dick about it, then yr girl should be fine about it. If she isn't, get a new girl, since that one is inconsiderate.
Just try your best, don't over stress your self because you don't have As or whatever all it matters is passing your courses to get that degree so you become what you are they won't care if you got the As they care about the degree itself.