I go through these periods too. I try to make a deal with myself that to sit down for 5-10 minute increments during these times and then take a break for the same period or so. I will even set a timer so there is a true promise to break then come back to it for only 10 more minutes. It is surprising how much can get accomplished in such a little period and will retain, then start to feel better about it and work actually gets done. If feel like building up to larger increments, 15 -20 min and then break, that's great but never more than 25 as start to fall back into losing motivation and retention becomes a problem over 30 min. I don't know if this method might help but I can definitely understand the frustration.

As Happycheeks mentioned, sometimes too, I can put things off not just because I don't understand it/its difficult and a tutor can help with this, but also because I am bored. I have to try to change it up, change my study patterns (reading aloud, walking and reading, or writing/typing things out), getting together with a study partner or a group, or using technology/computers to help engage me by finding additional material to make it interesting and tie it all together.

Hang in there and good luck finding what will work for you!!