Do you want it to last forever, then act like it. Commit yourself to honor and respect your partner. I cannot stress enough that you must make this your lifelong choice. And it must be mutual. If your partner is lukewarm, move on. Keep the love but lose the partner. You can do better.
Intimacy is not just physical sexual activity. It is also feeling safe with one another. It is touching hands. It is smiling across a crowded room. It is the never-ending enchantment and excitement you find in one another. It is laughing at each other's joke when they aren't funny. It is the warmth that suffuses your being when you think of them.
Sex should never be anything but lovemaking. This act of sexual intimacy is powerful because it is deeply embedded in our genome. It is the way we guarantee our immortality even if it does not lead to viable offspring. I personally do not understand how a person can have sex with another to whom they are not deeply attached.
Humor has to play a major role in Endless Love. You have to laugh at the same things. You have to be able to fart in front of one another. You have to be best friends. When you fight, you must fight fair. You have to be able to air out all your grievances without the fear of loss. You don't need to agree on everything. Indulge one another.
Jealousy and envy have no place in an Endless Love. If you are confident and made a good life-partner choice, you have absolutely no reason for jealousy. Jealousy is an insult to your partner's moral values. If you think so little of yourself that jealousy rears its ugly head, then you need to back out of the relationship and seek counseling.
By the time you are 16 or thereabouts. You already love the person who will be your partner in Endless Love. You may not have met them yet. But you know how they look, the feelings you will get inside. You already know the comfort and warmth. You already love the safe place they provide without being asked. They will be like that song you love.
You and your Endless Love will be a rhyming couplet in a corny song. You will be a Napoleon cognac they will be truffled Duck Magrete. There will be no question that you two belong together. This makes a base for an even greater love, a child. Nothing is like seeing you progeny the first time. Then comes the grandchildren, the greatest thank you of all.