You won't understand these reactions, or how they are related to the incest. You won't understand why the hurt is so enormous. You won't understand why something that happened so long ago can still affect your partner so severely.
Here's the thing: you don't have to understand. The only thing you need to know is that your partner is in pain. The pain is not going away, it's actually getting worse, and this whole situation is frightening. You don't know what to do. The specter of abuse is endangering your relationship.
Your job is to help your partner get help to recover. Learn the triggers that cause your partner pain and make sure to avoid those triggers at all costs. Triggers can range from words, tone of voice, touching or grabbing a body part, arm motions, sounds, music, the time of year, holidays - the list can seem endless.
It's understandable that your partner's pain seems so enormous that you doubt anyone can help. But there are millions of people in the world who would tell you that with a proper, qualified therapist, your partner can be healed. Your partner can laugh again, dance again, and feel joy again.
You need to know that healing is possible. You need to know that you will need to give more patience and understanding and love and compassion than you thought was humanly possible. But it is possible, and you have to provide this support. There are many resources to help guide you through the process.
You also need to know what does not work. Nagging, shaming, and lecturing do not work. Telling your partner to "just try harder" does not work. Implying that it is a question of strength of character -- you have it, and your partner doesn't -- does not work, and this tactic is horribly wrong. Child abuse affects every part of the body and mind, and only a psychiatric doctor can guide your partner to health.
A visit to a marriage counselors will not work. Going to see members of the clergy won't solve the problem. You will need to see a highly trained specialist.
Recovery will take a long time; that's a fact of life. If your partner doesn't like their therapist, remember that therapy didn't fail -- one therapist failed. Find another and try again. And again. Never never never quit.
If you're deeply spiritual or religious, here's another thing you need to know. Turning to religion instead of pursuing therapy does not work. However, many people use religion as a source of strength and hope to augment and enhance their therapy, and that does work. Do not consider therapy to be a rejection of religion. Therapy is a method of healing, and healing is a sacred act that is celebrated by every major religion. Nowadays, therapists know how to heal survivors of child abuse. That knowledge is a gift from God, and pursuing therapy is a celebration of God's gift.
The last thing you need to know is that this is your ultimate test. You are Winston Churchill during the Battle of Britain. You are Martin Luther King giving his speech in Washington. You are Susan B. Anthony facing a rioting mob. How you handle this crisis will shape the rest of your life.
This is your time. You are strong enough. Your love is strong enough. You can do it!