Relationships and growth?

How much of our capacity to love another, be in/have a  mature relationship with another is based on our mental age? I know that the majority of people today have the mental age of a sixteen year old. That an adult mentality is becoming more and more an uncommon thing in our society. I myself wonder what my mental age is. Most days I don't think I come close to being an adult. Just an angry teenager with too much responsibility. Its rather upsetting. Our whole society seems to be pushing for and trying to keep us all at the level of teenagers. How can we ever hope to accomplish anything with that mindset? How are we suppose to have a meaningful life if we are all kids? And if we have kids of our own how are we supposed to be their parents and not their friends when it matters?
How can we have a positive relationship with another if we are not adults. If we are all teenagers mentally, then how can we overcome the societal programming that influences much of this. Our society is such that it is seemingly ingrained in us as we age that we are to be with one person and that they must  never stray in any way. We receive so many mixed messages as we grow. They bring about insecurities, mistrust, jealousy, anger, hate. That by the time we are a so called adult, it seems almost impossible to have a healthy open and trusting relationship with another.
I don't trust my partner. They have not the ability to be forthright and honest. Their upbringing has instilled into them an inability to be honest for fear of a negative reaction. That dishonesty was always the better option. Their past actions have been in extreme poor judgement and as there has been only some improvement, I still can not trust.
Then there is the realm of sexual expression. There are monogamous  polyamorous, swingers and many more ways to relate to another. How does one grow, or not but say they have if only to be with a certain person, to allow multiple partners. I myself have no issue sharing my partner with other females. And while I have many issues with females, due to experience's of my youth, I have no problem with her being with others of her sex. But then if its a male I have many issues.
I have asked myself why this is. I am an insecure person. Doubts and fears of my partner staying with me. Some of this is me, some is comments or conversations we have had. And while I don't think she would leave me for another (and I don't think there is anyone who would put up with her for to long), I cant help but wonder. I know she has many issues with men, due to a horrible past with them. And she views sex differently then me. I ascribe more importance, sacredness to it then she does. Sex is just sex to her. I have a hard time having sex with anyone I don't have an emotional connection too. Which is what causes some of the tension we have with this subject.
I wonder if as I have aged, have I grown mentally more adult instead of remaining a teenager. Or have I remained a teenager but learned to be more functional in a so called adult world. Could I now if the situation was to arise, allow my partner to have sex with another male. Or would I still deny this. Or allow it thinking I could handle it, but then afterwards go off the deep end and destroy our relationship because I let something happen that I was and may never be capable of accepting. And why can I not accept this. Is it societal conditioning at work here, or is it a more primal instinct. We are very much at the mercy of our reptilian brain it would seem.
Why does this have to be so hard. We should be adults. We should know and believe, that if we are in a relationship with someone. That they chose us for a reason. That while we may not be able to satisfy them in every way, this is okay and we shouldn't try. We should not force ourselves to be something or someone we are not for this could lead to the end of are relationship. If we can not or are unwilling to satisfy whatever are partners need to be happy, then we should not deny them this. We should be supportive and help them to find a way to satisfy their needs. Even if this means it will be in someone else's bed, they have still chosen to be with us. That at the end of the day and for as long as it lasts, they will be with us.  And this is so hard to accept, to let happen. But damn if we don't need to face these fears and insecurities and for once in are short lives we need to grow up and be a better person then we where before. For if we don't grow and become better then we where before for someone we love, then whats the point of being in a relationship in the first place.