It is energy for defense of something you love when it is threatened.
If you don’t love something at all, you are not angry when it is threatened. If you love something a little, you get a little angry when it is threatened. If something you love is an ‘ultimate concern,’ if it is something that gives you meaning in life, then when it is threatened you will get uncontrollably angry.
When anything in life is an absolute requirement for your happiness and self-worth, is is essentially an ‘idol,’ something you are actually worshiping. When such a thing is threatened, your anger is absolute. Your anger is actually the way the idol keeps you in its service, in its chains.
Therefore if you find that, despite all the efforts to forgive, your anger and bitterness cannot subside, you may need to look deeper and ask, ‘What am I defending? What is so important that I cannot live without?’ It may be that, until some inordinate desire is identified and confronted, you will not be able to master your anger.