How to relieve very severe constipation?

Diet vs medication. There are lot of good medications available to help with constipation. The dose and safety will depend on the age of patient. Three steps to treat constipation: good initial clean out, then maintenance and then adjusting diet to help wean off and decrease use of medications. Some patients may need medications for a long time and that is okay depending on what meds they are on.
See physician. If you are suffering from severe constipation and so far nothing has helped, please see your physician about it. He may need to run some test on you and treat the main cause eg hypothyroidism etc. In the meantime, drink plenty of water!