Well this is where you can ask yourself some questions. The shoulding applies to you too. You “should” not be any different from what you are but being very insecure might not be helpful in having your needs met. You can try to take steps to reduce that insecurity. Or you can continue being insecure and pursue a relationship with someone who is willing to go the extra mile to not trigger it. That may not be easy, but you also might get lucky. Are you okay with that, or does that also make you feel insecure?
Put aside the convoluted projections, strategies and what-ifs. You’re just trying to have your needs met, and he is trying to have his needs met. It’s up to you to figure out if you can do so in unison or not. But the thing is, you WILL find out eventually. You can anxiously wait for the conclusion, you can play around and have fun while you figure out the conclusion, or you can get to the conclusion as soon as possible. But the conclusion is inevitable regardless of how much time you take to get there. Trying to force him to be any different won’t change that. You pretending to be any different from what you are won’t change that. But I can say that in general, the more people stray away from what is true, the more they tend to suffer.
Girl you’re 21. You are so, so, so young. You’ll have plenty of time to fall in love for the rest of your life, get your heart broken beyond repair, rally to find your true soul mate again and so on. I get that it’s basically impossible for you to really see it that way even if you probably get it intellectually. My 21 year old self would tell this current 41 year old me to go fuck myself for not understanding anything.
Take the hit here, see what happens. Maybe he will think you’re a bit crazy or maybe he’ll be understanding. If your relationship ends, cry your eyes out, scream and wail and temporarily believe everything is lost. Whatever you’re feeling, express it with your whole body, dare to be dramatic, just don’t take it out on others. Dance like nobody is looking and cry. Then you’ll go on with your life, you’ll have learned that well actually, everything isn’t lost and you can live with this. Then you’ll find someone new and you’ll A: be a bit more confident because you’ll know you can survive a break up and B: you know more about your needs and values, and you’ll know to communicate them early on so there’s mutual clarity. But maybe he’ll think that your insecurity is cute or so. Maybe he is willing to work with it. Then I HIGHLY recommend that you both look into Non-Violent Communication. Take a course together and you’ll form a very strong bond, and you’re both better equipped to communicate with each other skillfully. This will help with your insecurity too.
Again, you’ll find out eventually which it is anyway. I get that you’re scared and no amount of “you’re so young” isn’t going to make you feel better about it. But you’re scared of something that hasn’t happened yet. Right this moment, is something so terrible that you can’t handle it? Do you have a roof over your head? Do you have people you can talk to? Do you have access to basic necessities? Right at the moment you’re reading this, are things bad? If the fear about future starts asserting itself, always ask yourself if you’re okay right this moment, or are you just getting anxious over something that hasn’t happened.
Also think of it this way: how wonderful would it be if you could just fully be as you are with someone without the need to hide anything? With dishonesty, you definitely rob yourself that possibility.