The solution is simple: if you are not comfortable with it, do not try to force yourself to do it. Or put yourself in a more comfortable situation where you feel more at ease. Watch a dirty movie or read some erotica to help get your mind in the mood.
That's okay! You just obviously don't feel the same connection over text, which is fine! But if you still want to, just imagine you're there with him and type what you would say to him. When you think of a texting conversation as a regular conversation with your voices voicing the words, it's not as scary :)