If the anger management gets worse over such a lengthy time frame, then there are several options you can take.
The first, which most people do try and do, is to set up a joint therapy session, whether it is with the same person who does the anger management therapy for your spouse or a completely different person who can deal with joint therapy.
By doing this, your silently letting your sprouse know your still there for them, as alot of people do grow angry to push people away out of fear of something and, as such, this could help both of you gain a better insight into what the problem may be.
The second option you have is to go and see the person who your spouse has been seeing without them and asking them to explain to you what they've been doing to help your spouse, maybe explain to them that it's grown worse. They may have found a topic which is hard for your spouse to speak or think of and as such, lashes out in anger.
The fourth option is to suggest a new therapy office or more sessions; depending on if your spouse sees someone alone or in a group, maybe a change of sessions could help them. Alot of time anger comes from fear or hatred of themselves or something that they have experienced and if they meet other people like them, then maybe it can help.
The fourth option, which many only take as a last resort, is to tell your spouse, calmly, that if things don't improve your going to leave, or maybe leave and stay at a friends or relatives home; tell them your leaving to give them a chance to sort themselves out.
As bad as it is, sometimes what people need is to see those they truely love and care for, leaving them. It can cause anger at first but if they truely care, then it can break through to them. However, I would only recommend that as the very last option to take.