I'm assuming that sex is not the only reason you are in this relationship. Otherwise, I doubt you would even bother writing for advice or calling this person your partner. When someone you love is experiencing pain, it can be hard to offer the support they need and sometimes your own needs fall by the wayside.
Severe injuries can be very difficult to accept. Injuries that incite chronic pain force the sufferer to face new obstacles and challenges, and accept new limitations. Your partner may be mourning the loss of dreams or expectations that they had prior to the injury. You may also need to grieve the fact that your partner will never be able to physically accomplish all that they had originally done or planned to do.
Today there are many options for individuals learning to live with serious injuries, including physiotherapy and pain management, and not all injuries need to have a permanent impact on one's life.
You mentioned that your partner's pain flares up - is this a regular occurrence? If so, perhaps finding ways to work around the injury during painful flare ups will benefit you and your partner and lessen your limitations.
Have either of you considered non-penetrative forms of sexual activity? One recommendation that I have is Sensate Focus Therapy. Sensate focus consists of several steps, performed in and out of therapy. While it is often prescribed for sexual dysfunction and other disorders, it may be beneficial to you and your partner, as the first three stages all involve non-penetrative sex. The stages build intimacy and allow partners to find other ways of arousing one another. You say your partner feels you only ever want sex, so intimacy-building could be an important step towards regaining trust. Your partner is most likely already dealing with a lot of physical and psychological pain, and may wonder what value they have to you, if they cannot please you the way they used to. Suggesting alternatives to traditional sex could be very beneficial to rebuilding intimacy.
Another suggestion is to check out Dr. Annie Sprinkle’s book, Spectacular Sex. She discusses several different kinds of orgasm and explains how they are achieved through non-standard means.
Make sure to reassure your partner that he/she is important to you. Try empathizing with your partner about their situation. Address the emotions both of you are having in an honest, open way, without being accusatory. Once you regain emotional intimacy, sexual intimacy won't be far off.