This was hard to watch, again. I think anyone who has gone through a divorce they didn't want understands, to some degree, the sadness and anger that Betty was feeling about Dan's betrayal and his moving on with Linda. Most people, though, move forward. They may never forgive their former spouse, or have warm feelings towards the stepmother that gets involved with their children, but they find a way to move forward and find something to look forward to.
Betty got so stuck, so hyper-focused on how wronged she had been, that she couldn't move forward. Part of that was the drawn-out nature of the settlement, and the legal trickery she had to deal with, exacerbating her feelings of helplessness. Part of it was the lack of any real support, emotional support, in her life from anyone, Dan, her parents, etc. I think some of her friends tried, but eventually she wore them down. She truly was mentally ill, but she had gotten herself to a point where she felt she could trust no one, and no one understood her.
The day of Dan and Linda's wedding, with Betty looking through her own wedding album, was pretty realistic. Lots of women probably treasure their wedding day as one of the biggest days of their lives. And now those memories, because of the way the marriage ended, are tainted, and especially when the spouse is moving on and marrying someone else, looking back at when it was YOU being in the wedding, well, it's sad, and normal, and also, maybe a good time to kiss all of that good bye and say, time to move on.
What an utter tragedy.