Pastafreak's right, the secret is, indeed, in the word 'university'. If you are studying with the OU, presumably you are hoping to get a degree - a qualification which is testament to a thorough knowledge of your subject, perseverance and to academic rigour.
The fact that most OU students are doing other things, too, like working and looking after families, makes it doubly hard and everybody who succeeds deserves admiration.
I have studied with the OU in the past and now I am a tutor. My advice to you is, if your marks are going down and you don't know why, contact your tutor as soon as possible and see what help s/he can offer. Sometimes it's possible to get a bit of 1 to 1 tuition to get you back on track, or maybe you're just missing something obvious somewhere. If that doesn't bear fruit, contact the regional HQ and ask for Learner Support. Discuss your problems with them and, again, see what help can be offered. Sitting worrying about it on your own is unlikely to do you any good at all.