Having all those health issues and treatments may have contributed to him experiencing depression and also feeling a loss of control which is something, from what you have described of your father, he battled with his entire life.

Anniversaries most definitely can be painful reminders, I have them myself so I definitely know the challenge. Each time you experience it, it's a chance to mourn something you have not mourned "yet" too and work your way towards finally developing some peace and acceptance within yourself.

You put in a lot of effort to have a relationship with your father despite his ongoing dysfunctional behaviors. What he ended up choosing to do had nothing to do with you or anything you failed at doing either, he made this choice on his own and often when that happens, as you know, it's very hard for others to accept and there are always those questions of "what did I miss that could have changed this?". Well, people can suffer and never be able to articulate it to others or even themselves. What your father did was "his" choice and "his" way of taking control over his own suffering. There was always a part of him that despite all your effort to love and help him, he kept to himself.
How he developed happened long before you were there in his life. He had deep problems for a very long time and he simply was not able to reconcile these problems and that was not anything you could have really changed.

So when you have to once again deal with an anniversary, as bad and traumatic his choice was, please make sure you consider the quality you "did" bring to his life. He just was never capable of giving you what you deserved and that was something that happened in him long before you were around.

Truth is, we ALL are designed to navigate and if we have to face challenges in our environment when we are young, we have no choice but to learn how to navigate around whatever may be unhealthy or toxic in that environment. Well, your father did not "know" how to be a father and a husband, he most likely was raised in dysfunction and in his generation a lot of men used alcohol to help them manage. A lot of the men were not allowed to have emotions so what they did was they tended to be angry a lot too. Often sitting and "feeling" was something they simply found unbearable, feeling vulnerable can also be something "unbearable". This could be a big part of why your father chose to do what he did. This did not have anything to do with his love for you and he was probably at a point where it just got so unbearable for him that he chose to take his life not even thinking about the impact that would have on you.

This is always so hard to understand for those left behind to understand.