How can I improve in sex?

You don’t need to ask your question twice :)

You just asked this. And remember to lead with the question and not a one-word exclamation that can be answered many ways.

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Ask her!
She’ll tell you what SHE likes, not what someone else enjoys.
Besides part of building intamacy between two people comes from learning about one another’s likes and dislikes.

You should write down all of the suggestions, plus notes you’ve taken down from answers on other sites. At night in bed, whip out the notebook and tell her that you’ve written down tips from all of your old girlfriends and your aim is to give her pleasure this way from your past experience.

I know that the Guide to Getting it On is a standard gift at the bachelor and bachelorette parties I’ve been to. You may want to check it out.

Have it with your girl and ask her what works for her and what doesn’t.

How do you know you aren’t any good? If she says you aren’t, then ask her what would make you better and then do it.

Sex is a form of communication, like most human endeavors, and you have to talk to each other about it if acts fail you.