Like all the above comments have suggested, try therapy because these issues seem a bit more deep-rooted to fix all by yourself. And please do not be ashamed of disclosing this w ur SO or your family. We all have feelings, we all sulk regardless of whether we r 10 or 40. Money is an issue for most when it comes to therapy but mental health is as important as physical health so scraping together some cash for a few sessions will serve u well in the long run.
And i get y u would be reserved. The decision u made 15 yrs ago is probably still stuck w u and u dont know how to get out of this. Maybe try initiating conversations, being more active in a group conversation or just ask about everyon else if u dont feel like talking abt urself. But dont try to imitate ur sister. You being u will be more than enough because all of us like and can definitely sense a genuine person.
One last thing, dont put off ur happy moments with ur SO just because u dont have many friends. Friends come and go, none of them stay forever and losing the truly valuable moments will cost you. You dont want to end up regretting this 15 yrs later so if ur attempt at socialising fails, just dont give a damn and value ur existing relationships.