You must live life on your terms and do the things you enjoy doing as much as you can. This will help take your mind off of your chronic illness and better deal with it.
Next you want to set goals for yourself. These goals can range from anything to exercise to nutrition. For example, if you have arthritis, you need to strengthen the muscles surrounding your joints and take supplements to ease the pain. Don't just tell yourself need to do this. Make weekly workout plans and days you will do them. This will put you in charge of your treatment.
Be ready to face your feelings. The physical symptoms of your condition aren't the only problems you're going to face. You will face mental emotions like anger, regret, depression, and fear. Try going to a support group to help with this.
Lastly, visualize the type of life you want and go after it. Strive to achieve all of your goals from pursuing hobbies or having an active social life. Don't let your illness define or confine you.