It does not really sounds like mental health problems. You sound very healthy to me, just not self assured/you don't take yourself seriously, and you pay too much attention to what others say about you but don't ask from them what you want for yourself.

Everyone needs attention and praise. But you both keep ducking and, not asking for it. Where's your anger when idiot supervisors judge your work having never seen any of it? Why did you not ask, "Hey, when did you see my work that you have decided I'm not good enough? I can beat any of your regulars around the block all day!"

Your cousin makes scenes, calls attention to herself and everyone comes around. You have to call attention to yourself but in ways you want. Have a little family party to show off your house. A barbecue in your yard would certainly be much more fun than the drama in hers? But maybe not with those particular people. Just because they are family, doesn't make them friend-material? Do you actually want to be around any of these people? Maybe have a little party at your house and invite a few people you work with?

You aren't in elementary school anymore, don't have to do things the way you think the "adults" want them because they're bigger than you and can hurt you. I still remember when I got to high school and college and walked and ate too fast! My stepmother was constantly telling me I was too slow! Our perceptions as children, are children's perceptions. We can change those when we get older? No, you haven't had the practice socializing a lot of kids get as they're growing up but you can practice now, can start now learning to get to know people and seeing if you like some and some like you, etc. You don't have to prove anything to have people like you, you don't have to be good at your job or not "steal"/share their toys; one of my favorite greeting cards says:

Beware of friends turned into lovers
They'll steal your heart and then your covers

Friends want friends to share their toys with, to be proud of and celebrate. Family, not so much :-) Join some groups that look interesting to you because they're interested in something you're interested in. Does your work have a baseball team? Make the suggestion they form one (if you like baseball :-) and get on the committee to design the teeshirts or caps, etc. Does a group of coworkers go out after work some nights, ask if you can tag along but make sure you're in there talking to people, getting to know them. If it's scary, you can leave early, claim a previous engagement and tell them outright you're leaving and goodbye, etc. instead of slinking off, feeling like you weren't wanted. To heck with what other people want, what do you want? If you want friends, you go out and act friendly, that's your only job, not to decide if others like or do not like you, you can't control that, all you can do is decide if you like/do not like another and the activity you have chosen to do.

That's why I say you are healthy. You went to college because you wanted to. You bought a house because you wanted to. You took a job you wanted and enjoy. That's what makes someone healthy/unhealthy, if they are working on what they want in life, not on what someone else wants or they think they should want or whatever. But for your personal life you need to start stepping up and doing that too; acting like and even saying, "I really like you, I want to be friends with you."