I think the reason that this relationship feels different to your first, is because you have matured, you've experienced and you just aren't as hormonal as you were when you were a 16-year old.
It's great that you mentioned that you're an insecure person..because i think this is the root of the problem. You feel like your relationship isn't right, merely because of the fact that you aren't feeling "insane" over this guy. this person is making you happy, and you say you love him..so just enjoy it.
people say you never get over your first love, maybe that could be another explanation for the way you're feeling.
As an aside, i wish you the best of luck. Congratulations on finding someone who makes you this happy!
"We all take different paths in life, but no matter where we go, we take a little bit of each other everywhere."