These good friends did reach out initially. Presumably they have also reached out many times in the 20 years, as you did to them, otherwise you would not still be friends.
It's harsh, but it is true that in the first few weeks, people make a huge effort after divorce, bereavement, leaving a relationships, but then their own lives go on. Remember, if you are in middle-age, the chances that they don't have adult children with huge life issues, parents who have chronic ill health, cancer, their own bereavements, cost-of-living stress is probably nil. Most of my friends have one quite large life problem right now, so whilst they can rally in a crisis, they aren't going to be able to keep that up.
You do sound angry at them, but your expectation of them popping over is probably unrealistic, more realistic is dropping them a message, asking if they'd like to get together soon, or setting up a social meet. They might also have things to off-load or want to get out more or have some fun, it is unlikely they are sitting at home having just a wonderful time without you- this is a narrative you have told yourself, but it's unlikely to be true.
It is also the case that you can't have all your eggs in one basket. That doesn't mean you have to rush out and make new friends, but it does mean that now you have got out of this horrendous relationship, you will have a lot more time and energy on your hands and it may take a while to know what to do with it- therapy is good, and then after that it depends whether you want to work, do more socializing, travel, study, or meet up with friends, new and old. It is hard work building a new life, I've had to do it myself after my husband's death- it includes my old friends but also new opportunities, or reconnecting with friends who might have fallen by the wayside a bit. It's hard though, and I do get what you are saying, you are feeling tired and exhausted and need a hand up in life. So, take the time to be very kind to yourself right now, just drop a few messages to your friends if you feel like it, send a couple of Christmas cards, attend a carol service and over time, that new life will start to become more visible as you go towards it.