Our understanding of love and romance is changing, and as it does, our understanding of what makes a happy relationship is changing too. People are becoming more open to the idea of alternate forms of romantic relationships, and given good communication and open hearts and minds, people who care about each other can find ways to express that affection in ways that both of them desire.
Second, the happiness of an asexual person is not dependent on sex or on whether or not they’re in a relationship. What makes an asexual person happy is unique to them and them alone. They may be happiest when they’re single, or have a few very close friends, or have a platonic partner. Their happiness might depend on how many house plants they own, or the time they spend reading books, or the work they do at their job.
While the thought that not wanting sex may mean you’ll be alone for the rest of your life may be daunting and often terrifying, it is not true. There is always love and affection to be found in this world, and only you can decide in what form you wish to seek it.