There are couples who have been together for ten years and have never said out loud what they enjoy. Not out of unwillingness. Out of discomfort, out of habit, and out of fear that the other person will take it the wrong way.
Because that is the real problem. Saying what you want can easily sound like saying what is not going well right now. And nobody wants to hear that, and nobody wants to say it.
Why it feels so difficult
We weren't taught the words for it. You learn how reproduction works and how to prevent an STI at school, but no one teaches you how to tell someone you love what you enjoy.
On top of that, the conversation usually starts at the worst possible moment: in bed, afterwards, when something just happened that didn't work. Then everything you say sounds like a judgment.
Four things that help
1. Do not have the conversation in bed
It sounds contradictory, but it is the most important advice. In the car, during a walk, while doing the dishes. Side-by-side instead of face-to-face, and with something else to do. That makes it feel less like a confrontation.
2. Start with what does work
"I enjoyed it when you recently..." opens a conversation. "I don't like it when..." closes one. Both provide the same information, but the first makes the other person curious and the second makes them defensive.
3. Use something outside of yourself as a prompt
An article, an episode of a show, a toy you saw somewhere. Talking about something external is easier than talking about yourself. "I read something about..." is a safer start than "I would like...".
This is also why people suddenly find themselves talking to their partner after a night with friends. Not because they learned something new, but because a prompt has emerged.
4. Don't expect to resolve it in one conversation
This is not an issue you resolve in a single evening. It is something that gradually becomes more normal, by touching on it briefly more often. Five minutes that didn't end in a disaster makes the second time a lot easier.
If the other person doesn't want to engage
It happens. Someone who gets startled, shuts down, or makes a joke of it.
Usually, that is not a rejection of you, but of the moment. Leave it be for a while and briefly come back to it another time, without it becoming a Capital-Letter Conversation.
If it remains undiscussable after several attempts, it is probably not about sex but about something else. In that case, a relationship therapist or sexologist is a better conversation partner than an article.
Why it's worth it
Because the alternative is that you both keep guessing. And guessing works well enough for years, until it doesn't anymore.
Looking for a prompt? Toys that you use together are a great one: the conversation is then about an object instead of about the two of you.