Writing is a skill that improves with time and practice. The best way to 'sext better' is go onto sites that host erotica content, such as Literotica, and read through some of the top rated stories. When the sex happens what are some key things you notice? The descriptive nature of the act, the passion from the character's POV, and the reaction to the partner. The same concepts can be applied during ERP. You want to first learn to be descriptive with the actions you are doing. Don't just say 'I kneel down before you'. Instead opt for a description that not only signals what you're trying to do but gives insight into how your character you're playing is feeling. As an example:
[Her knees tremble slightly as her eyes match his. The words echoed in her minds and she felt the sudden urge to obey him. With a graceful curve of her muscles she slowly descended towards the hard flooring, the cold wood pressing against her flesh as goosebumps raced across her skin. Her body lowered itself upon her legs and she felt her weight spread out to allow her the ability to be comfortable in such a compromising position. Her eyes fluttered upwards, heart racing with desire for her Master, as she met his eyes. Though she did not utter a single word, the words she wanted to speak in this moment were hung in the air. She wanted to hear his approval, the desire to hear those cherished words screamed in the silence surrounding them.]
Once you feel more comfortable with the idea of being descriptive with your posts and fleshing simple actions out you can work your way into describing complex actions during a scene. You'll find that you are at a much better footing to express what you are wanting to convey than you were before.
