The OPs gave you good advice, but I would suggest that you focus on your first observation--that you have improved your comfort level in social situations. Most people were not born with all the social skills that you need to feel competent and comfortable in EVERY situation. You learn these things, either by mistakes or by "study," like anything else. Most of it comes from learning about yourself, what you like and what you believe.
One of the other things you learn as you mature is that what one person says may not be the whole story. The "other girl" you mentioned may believe and practice what she told you, but that doesn't mean that everyone else does. For instance,
"I did have a discussion with another girl who told me that apart from what you do alone with your partner doing more adventurous stuff with other people or couples was what made you feel alive (in her own words). She told me that in the end monogamy doesn't work and the majority of people after a while in their marriage just stop wanting to get intimate with their spouse."
If you read the posts on this board, the majority say something entirely different. The marital bond is strengthened by playing and experimenting. The ultimate reward is in how it makes you closer and better with your spouse.
So, I would read more, get some "second opinions" and take what the girl told you as one person's opinion, not the gospel.
Also, your command of English, not your mother tongue, is fantastic. A lot of the stuff I read is not close to as good. Congrats.