I think the biggest reason authors struggle with after-confession romances is because they wrote the story with characters that are just a vehicle for an emotional event (confession). It may feel like a character driven story but it's not, it's event driven. Without the event the characters feel empty and meaningless. It's only after the event (confession) that the author realizes they need to give characters personality and individual motives. Which is why they don't "feel" right. What you had before was a character without a personality and now the author realizes that's boring and tries to fill the gap.... and does so poorly.
Though this might super impolite to point out, these kinds of romance authors may have never been in healthy, happy long term committed relationships. They are writing romance based on ideals and wish fulfillment. I'm not saying that's bad or has no place, but it lacks.... the realistic touch. How two people act in a romantic relationship requires some understand of... well... how couples actually interact.
For the first, have some idea how your characters behave outside of a romantic setting. What are their aspirations? What are their flaws? Their strengths? Where are they now and where do you want them to be by the end of the story? Know them as people then add the romance in. Once the romance is added, ask what changes and what stays the same. Figure out where there will be points of friction and where they'll help each other be stronger. It doesn't have to be in-depth, but have a general idea in advance, to give yourself a guideline going forward.
For the second, you don't have to be dating or married to write good romances. I think it helps, but it's more important you know what makes up a good relationship in general. For this I can give you a very solid piece of advice:
Good romantic relationships have a solid foundation of friendship. They don't have to start as friends, it can start as a physical attraction. But if friendship doesn't grow alongside the romance, when the physical attraction fads the romance will die. It's being friends, finding the other person interesting without being kissy-kissy faced, that keep a relationship going even when he always leaves his underwear and pants on the floor or when she never cleans up her hair clog after a shower. It's friendship that creates laughter and inside jokes and it's also friendship that powers a couple through their flaws and irritations. When you write a romance, you're also writing a friendship.
I don't know if I write good romances (you can read my stuff to find out), but I've been married over 15 years. I love my husband to bits. He's my best friend. I tell him secrets I won't tell anyone else. I express the deepest parts of myself in ways I won't do with others, including family or best friends. I'm my most "raw" when I'm with him, which can be both good and bad. The same goes for him.
When you think of your couple, can you imagine them being that honest and trusting with each other like that? If you can't, their relationship is no better than a fart in the wind. It won't last and will gross out everyone around them.
