This is because, at least in our case, it's hard to keep up the same emotional connection when there's huge distance between you. Every time you see each other, you have to get to know your partner again a little bit because things will have changed since last time. It's not uncommon to feel like something is "off", even if you can't place it.
When you don't get to see each other regularly, being back together can feel like an invasion of personal space. In order to move past this feeling quickly it's important to acknowledge it – each and every time. It's not easy going from never being touched, or feeling any human contact, to having it 24/7. I know it definitely makes me uncomfortable and even slightly claustrophobic, but this negative feeling is usually dissuaded after a couple of hours of non-sexual physical intimacy – like just holding hands.
Taking the pressure off being sexual with your long distance partner is crucial in actually maintaining an enjoyable sex life. Sex anxiety is a real thing. Even when it's somebody you've made love to countless times, a sense of nervousness can come with infrequent sexual encounters. You can feel body anxiety, performance anxiety, generally just worry about being "good" in bed... usually these feelings are because you don't feel like you have the luxury of being "off your game".
Sexual chemistry between partners in a long-term relationship should therefore be kept up in other ways when you're apart. The most obvious solution in our 21st Century connected here would be making NSFW videos and sending nudes. However, a lot of people aren't comfortable with this. My husband, being a security-conscious Army officer after all, has serious privacy concerns about the exchange of these types of sexy media and prefers that we don't do them. I personally don't care about somebody intercepting my naked body, but when your partner isn't okay with it, it's fundamental to respect their reasoning.
Sexting with just text messages is a more comfortable route for those who think videos and photos are too much. It lets you build up sexual intimacy at a pace that works for you based on conversations you mightn't even have in real life. You can talk about your fantasies, fetishes, things you'd like to try, the things you miss about each other's bodies, and so on. It's less awkward and there's no worrying about good lighting or angles.
I recommend Whatsapp for this because its end-to-end encryption is the most secure of the
mainstream messaging apps. We stay away from all other Facebook products for obvious historic leak reasons. If you're really nervous or want to keep your sexy texting really separate from your regular communication, an app like Wickr Me allows you to exchange end-to-end encrypted messages that auto-expire and self-destruct.
Often long-distance relationships involve different time zones and actually having live contact becomes challenging. This is why I recommend writing longer, story-like erotic messages that your partner will pick up at their convenience. Rather than sending short in-the-moment texts, take your time writing a few paragraphs. You might want to explain a start-to-finish sexual encounter you've been daydreaming about, for example. Another similar idea is to send each other links to pornography that you know you will both find arousing.
When scheduling contact is possible, there's good old phone sex. Remember that? Masturbating with somebody else while just talking and turning each other on is fun and very intimate. You can use dirty talk, act out a past sexual experience, or indulge in a fantasy. If you're really keen on a mutual-masturbation session, there are even sex toys out there (called teledildonic toys) you can control remotely via your smartphone. Talk about taking control of your partner's pleasure – this is the future of sex for anyone who doesn't get to see their partner often.