I agree to just focus on yourself for now.
I'd immediately detach from this group -send a quick breezy message saying something like you're having some phone free time or some issues have cropped up, take care everyone, and then leave the WhatsApp group?
Dragging things out or explaining/discussing will probably only affect your self-esteem negatively....if they even respond at all.
If anyone asks I either wouldn't reply or wouldn't give them any information, just confirm you're busy or send a smiley face. No messages or Christmas cards or anything. It will be a weight off your back and less emotional energy.
Of course you may meet new people in time. However, I wouldn't immediately start a project to meet new friends.
Firstly, you've been through a lot, and may come across as vulnerable or get in with the wrong types/users/predators?
Secondly, it is difficult meeting new people sometimes and I don't think you want to feel rejected again by more people!
Maybe it might be best to just build up a routine where you have some hobbies you enjoy and are good for your wellbeing (say exercise classes on YouTube could be free and done at home and are good for your MH). Or reading.
Like pps say treat things as a long term project. It may take 1-2 years but I wouldn't do anything "dramatic", just take time and small steps.
Get a home coffee machine or something and if you do meet people naturally, enjoy them but don't worry too much about chasing after people.
I've found it really good for me and my confidence improving my home environment, so I feel if I'm staying in it's a luxury...scented candles and being really diligent with tidying and revamping my home, and making it a real luxury staying in. I don't get FOMO as it's too nice in here! Maybe this is an option? Staying in is new going out in many ways.
A lot of people are in the same boat just focussing on work and money and home and family, without having a so-called girly gang (not that theres anything wrong with that, but it's not like compulsory!).
Look at all of us chatting shit on Mumsnet at weekends with our "successful" lives ;-) A lot of us have all been through relationship and friendship issues and improving our judgement on things as well.