Some ideas:
1. Practise planning essays in time conditions. Lock yourself in your room, remove your phone, turn off pc, take out batteries from MP3 or put headphones in a different room, and then bring out the trusty kitchen timer, or stopwatch.
GIVE YOURSELF 10 minutes to write all of an essay plan and then STOP at 10 minutes. Even if it is crap!
2. Now take some time looking over it and fill in stuff you didn't get, checking your notes and everything.
3. Repeat exercise again in about 1 or 2 days. Rip up old plan and see if you can redo the same plan WITHOUT MEMORISING THE FIRST ONE.
4. Then try it again after 1 week, then 2 weeks.
Repeat this for how many essay topics you have. Never memorise the essay plan you had before, since the purpose is for you to recall ideas not the words or sentences you wrote down. Of course you can memorise the quotes and evidence...
If you've mastered this, instead of writing essay plans, write essays in under 50 minutes. TO A TIMER. When the time stops no matter how crap or how far you've done, that's it.
Just a suggestion, try it!