The only things doomed to fail are the promises you made in your 5th grade D.A.R.E. class. Relationships, on the other hand, can last as long as the partners involved are committed to making it work. My advice for long-distance lovers: have an end-date in mind and find ways to share sexual intimacy from afar.
I believe that LDRs are only sustainable if there is an expiration date that helps partners keep the end in sight. For example, after X months/years, partner A will graduate/escape from prison/abandon their wife and children and move to be with partner B.
Call me a sex addict (like my therapist does), but I also believe that couples need physical intimacy to survive. Sexting is a good way to test the waters of extending your sexual interactions. Share a fantasy or reflect on a memory of an epic sexual experience you shared. Try to exchange a combination of words, photos, and video clips. Allow your partner to reciprocate and take some time between messages to build anticipation and excitement. When you’re ready for the big leagues, try masturbating together over the phone or video chat. Your ability to get each other off knows no bounds.