It seems to be anxiety about relationships. It can be corrected with counseling sessions and medication if required.

"Hello! Thank you for reaching out to me. This sounds like a difficult thing that you're going through. Afterall, everyone wants that one special support system in their lives. Yes, it is common to feel more lonely after seeing couples in public, but a girlfriend isn't the solution to all happiness. It sounds like you have been dealing with a lot of issues and anxieties relating to socialisation. Since every case is unique, it is advisable to reach out to a professional and deal with these anxieties! You're not alone.

Hi... Being an introvert does not mean you can't form relationships or friendships. It only means that when you have a choice you would like to spend time by yourself, rather than with other people. It is more of a preference and less of a compartmentalized categoy. Even introverts can learn soft skills and be good at arts like public speaking, group discussion, leading a meeting, etc. Being attracted towards the opposite gender and even this fear of missing out is normal. The only problem is when it is limiting your daily life functioning.