Your problem seems to be rather unusual as I have used the eseries for a long time and have never had a memory problem like yours. Though a google search seems to suggest otherwise. I am suggesting a few things that you cross check and set on your phone which should eliminate these problems. This is how I have setup my devices and it seems to work on them.
- Check that your applications are installed on your memory card instead of phone memory. If not then please reinstall them there. Check settings for each application to ensure that it uses the memory card instead of phone memory. The only applications that I have installed on my phone memory are those that cannot be installed on the memory card. Check settings for your camera and music as well. Only your sms messages and your contacts are allowed to occupy your phone memory (as it is faster) everything else that is optional goes on the memory card.
- Check software version of your phone and update it if there is a new version available.
- WhatsApp: Check settings to ensure that all your messages and files received on whats app are stored on your memory card.
- SMS: Check sent items and drafts. Limit Sent items to 20 or 40 only.
- Email settings should be for upto a maximum of 7 days. Make sure it syncs your important folders only. I suggest 3 days for you right now. You can always increase it later.
- Email application should be updated. Check from here
Keep restarting the phone (to clear up the memory) and checking the usage after each important item (such as shifting from phone memory to memory card in whatsapp) so you know which one was the culprit and you know what to avoid in the future.
I hope the above items help.