I understand it's hard to concentrate sometimes when you have ADHD, but you can always start meditating when you are still in bed in the morning or going to sleep in the evening. That would ease you into the practice of meditation & it helps a lot.
As for meeting new people: you don't have to join any group to go for a walk. Just go by yourself & enjoy the beautiful day. That will bring smile on your face, the real genuine smile. Also when strangers smile to you, smile back at them - it's the simplest human interaction & it doesn't cost anything. I'm not sure what area you live in, but everywhere in the country they have lots of free events going on every week. Those are great places to meet people. You can quickly find out what's going on online.
Plus think about the good things in your life & be appreciative of them. I also think it's very helpful to have a "Happy place" in your mind where you can go to when you feel sad. This "happy place" could be as far back as your childhood: it's a time when you were truly happy by yourself. It helps a lot to meditate & go to this "happy place" when you're stressed out.
So, please, give these things a try & you'll feel better.
Wishing your the best of luck & prosperity!