You will have plenty of time to relax and enjoy quiet time while retired, so it is important to keep that in mind. Just because you have finished full-time work doesn't mean you are losing purpose in life. You should be proud of the accomplishments that you have made and excited for the new challenges yet to come.
Do you have a dream that you never had the chance to achieve while at work? Now could be a good time to make it happen, as long as it doesn't break the bank. Maybe you wanted to hike the Appalachian Trail, or even try something as bold as skydiving. Since you do have more freedom to enjoy the things you want, take advantage of it.
You also aren't alone, and should be sure to surround yourself with people of all ages for support. This is important, as hitting a big milestone in life will put you into uncharted territory. Others who have already made the switch to retired life may have positive input to help you deal with it better, and those who are younger may be able to help you feel young as well. Asking questions and reaching out are good and healthy things that could make the process even more fulfilling. The myretirement.org discussion board is a good place to start.
If you aren't ready to quit working completely, that's okay too. Everyone deals with their transition differently. You could easily work a job in the industry you are familiar with on a part-time basis, or you could try something you never dreamed of doing. There are many doors open to those in retirement, and that is one of the most exciting aspects of it.
A senior citizen, or someone approaching the “golden years”, earned the free time that is in front of them. If you view your situation more as an opportunity than a punishment, you are likely to get more out of it.