Lol I know I'm a lot younger than you (if your profile age is real) you but I know exactly how you feel! I've been with my boyfriend just over a year now and he goes to university in Cambridge which is far from where I live so I only see him at the weekends. I used to find it hard to sext and have phone sex because I felt kind of embarrassed and awkward and slightly ashamed. I thought for some reason in the abck of my mind he would see me differently or judge me or something but he didn't he loved it! I shouldnt have been worried at all.
I think you just have to really be in the mood, you have to be really horny and comfortable with saying dirty things. Think about your perfect sexual scenario and describe in detail what would happen...don't be ashamed or embarrassed...its so normal. loads of couples do it all the time :)
I hope I helped a little bit :)