What is the secret of a happy relationship?

There's no eternal happiness in any of worldly matters. Get married and start compromising. There are ups and downs, that's life. Hold up to your deen, always ask Allah for guidance and add the element of success (means Quran) in your life. There's not any easy answer to your question. Remember one very important thing; if you are in a haram relationship, like having a girlfriend or boyfriend, then remember that you can't ask for Allah's help into those matters, because He doesn't support the haram. That's the reason why most of such relationships don't end well. The relations whose base is immoral and forbidden, can't grow at all. Hence add the elements of purity and moralistic values in your relations. May Allah guide and help us all. Ameen.

Why do I have sexual desires at this age? How to control my lust?

To have s*xual desires at this age is normal, you may already be proned to excessive interaction with social media and movies which trigger you more into it, you may want to fall in a relationship or watch immoral content online to calm yourself. But the best ones are those who control this sensation and overcome it with patience. Every age has different problems and tests, there will be almost no day without any kind of trouble or test in it, so do not let Iblees win, don't make hasty decisions, ask Allah for help.

---

Don't fall for haram things, it's not the cure, it's a harmful painkiller, it will destroy you badly from inside, don't let a temporary need or feeling ruin your entire life completely, it's better to stop now than repent later. If you can marry, then it's the best thing you can do to help yourself. Try fasting, because Quran says that fasting helps overwhelm this feeling. Don't let it go out of control, because once it's out then you won't be able to get the control back in your hands. Good luck! Salam and Peace ❤