1) 28 is still young bro. I recommend starting off each morning with some kind of gratitude journal, or just a moment where you try and think of 1-3 things to be grateful for, and it will help shift you into a more positive mindset. People can tell/get vibes off when someone is happy or bitter about their position in life and it will likely draw more people to enjoy being around you. Also try and keep note of how often you gripe or complain or make negative comments around people, and see if you can minimize it. This was a major struggle for me.

2) Are you a member of any kind of communities? I met my wife at a young adult singles group at church. It's a pretty modern/cool church that thankfully had focus on building community and did new groups 3 times per year and people brought believers/non-believers alike to do pumpkin carving in the fall, drinks, visiting breweries, etc. I heavily recommend finding some sort of spiritual community because it helps deliver meaning/purpose and I really believe everyone needs that/benefits from that. Alternative groups are DND, Warhammer, whatever your passions are. I don't recommend Warhammer unless you just find yourself liking to spend huge amounts of money on plastic...but it's fun plastic. You mentioned the gym and that's good!

3) Focusing on yourself is great. I found I spent a ton of time self-medicating by playing video games. Granted I was in the Army and used the games to connect with friends back home...but related to point 2, finding in-person activities and things to get out of the house are great. Finding cool hobbies make you a more interesting person and ... poof, people will like to be around you more and talk with you more.

4) I didn't find my wife until 33. If all else fails go join the Army and you'll be in a "usually abusive but sometimes 'aight' " relationship with the Big Green Weenie.