I know firsthand that this kind of thing can later develop into really bad social anxiety, so let's try to pre-empt that a bit just in case. Don't feel too guilty about re-opening a line of conversation that you got bored of long ago. It's kind of normal for people to get bored of other people, and people will be pretty happy if you contact them again after a long time.

Do any of these accurately describe how you feel?

• People are actually fundamentally kind of boring; you exhaust their interestingness after a while. After a while, it sounds like they just repeat what they say over and over, and your conversations just become repeats of old things. Or not entirely repeats, but they just don't go very far.
• Even when you make a friend, it feels like there's still some sort of wall that you can't get across. They try their best to listen, but at some level it's like they don't really understand what you're saying.
• When you make conversation, you feel a bit pressured to respond rather than the conversation flowing naturally. Or maybe it starts out normal, but it develops into that over time.

If all of these do describe you, one thing that might help to keep in mind is that it's important to be not worry about offending people and be honest with what interests you and what doesn't. If you don't tell people what interests you and what doesn't, they won't be able to hold interesting conversations with you, because they don't know what is interesting to you.

At some level, you need to do some work to "extract" the interestingness out of conversations, whether that be dragging people into topics you like, or making people talk about whatever's interesting about themselves.