I'm a bit of a 'before and after' case! I've been horribly shy all my life and had no idea how to talk to most people - some people (not many btw) I seemed to feel a connection with but had no idea how to turn a quick hello at the shops or nursery into a friendship, so I pretended it didn't matter to me. I was also worried that it would affect ds1 as I spent the first 2 years of his life basically not leaving the house.
This all changed when my dh got a job abroad and we moved halfway round the world. We've been here for nearly 4 years now, and although the first 2 were tough, it really made me get out and force myself to meet people and forge friendships, even if I was terrified and couldn't think of a thing to say, I though just smiling and sending off a 'nice' vibe would be better than nothing!
The reason I made myself do it is because without family to fall back on as a social measure I couldn't stand the thought of my ds growing up as socially isolated and awkward as I was as a kid. Being shy has really affected my life and robbed me of a lot of chances and I just don't want that for him.
Anyway, after a while I found that over here it was easier to make friends than in the UK. Maybe because we all have this huge thing in common - being uprooted and making a new life for our children - I found it was not as hard as I'd thought to connect with people, although it does take a lot of practice! Sometimes just showing up and looking friendly is enough to make others want to approach you, and lots of people don't mind if you're quiet and shy.
By the way OP, I also got told when I was younger that I walked around looking 'stroppy'..this upset me so much that I trained myself into smiling constantly, which in retrospect probably made me look a bit deranged!
Obviously what worked for me is not really an option for you - moving abroad is quite drastic! - but two things I would recommend to make it easier. One, asking people questions is an easy way to talk, especially if you plan some in your head first, and two, realise that you don't have to be the life and soul - a lot of people really like quiet listening types to be friends with.