Your main job will be to support and encourage your partner, helping him or her continue as much as possible with the life that he or she loves, while recognizing that you are dealing with a very serious situation. Many caregivers decide that it is very helpful to find someone else with whom to talk about their own fears or worries. Consider meeting with an oncology social worker, or joining a family or caregiver support group.
If you were with your loved one the first time he or she had cancer, you have lived through this before, and it is likely that both you and your loved one will feel better in a couple of months as things settle down. Ironically, you have already gained the experience you need to adapt to the situation, develop a good working relationship with each other and the doctors, and establish a new treatment routine that will work for everyone.
If you were not with your loved one during his or her first experience with cancer, he or she may be able to tell you some of the things that did and did not work well in the past. As with all serious problems, this will not remain an acute crisis after some time has passed. You will, together, learn to live with this new reality, and your support can make a very big difference. Together, you will be able to get through this.