I think that people often latch onto nice people like you.
I dunno but I think that perhaps when you are lonely and actively looking for friends, you can come across as a bit needy and this might put off some of the decent people and attract the not-so-decent people.
When I first had dd I was in the same position. None of my friends had kids or were even married so I felt like I had to start again. I had just recovered from ante-natal depression and really did feel like a fish out of water.
I went to a few meet-ups with the NCT and with baby groups but most of the mums had been to ante-natal classes together or had known each other for years and it's really hard to make an opening for yourself in that environment. I didn't drive and so every week I would trek across town with my dd to NCT coffee groups. Then one week I plucked up the courage to have one in my house. I spent that whole morning cleaning and making little cakes. No-one turned up.
I've had plenty of disasterous friendships since which always seemed to be on their terms and never on mine. Like you, I wondered why no-one was interested in going out or coming to mine for coffee. Why was it that I always seemed to pay for lunch out? Why was I always making the effort?
It wasn't until I gained confidence, got back into my career and became comfortable in my own skin that I started to make actual friends. It's not easy and even now, I find that I am offering to have my kids' friends round for tea more than they do. I think that people are just too ready to take advantage of others when it's convenient to do so.
My advice would be to stop looking for friends. Do things for yourself for a change. Do you work? If not then volunteer for something. Take up a night class. Join a local drama group to boost your confidence. Challenge yourself. Do something that you wouldn't normally do. Do an unusual fundraiser (there are plenty of organised fundraisers for comic relief). Go out of your comfort zone and tackle your fears head on.
IME when you stop looking, as in love, you suddenly find someone. People can sense a confident, relaxed and interesting person. Have you ever found yourself drawn to someone? Or watched as one person seemed to lap up all the attention? They are people who are happy with themselves, who don't even need to try anymore.
We can't all be like that but if you lead an interesting life that is full of challenges you will become by default an interesting person who is brimming with confidence. So stop looking for friends, start focusing on turning your life around instead and you will find that friends will start looking for you.
HTH x