M.R.
A happy mom who can manage her stress and balance her life to include 'me time' is the best type of mom out there.
Do it! You'll be a much healthier person and your child will benefit in so many ways.
The first time you WILL feel guilty but commit to the free time activity you've granted yourself. It will get easier.
You are not abandoning or neglecting your child by attending to your own needs from time to time. You are actually modeling a great behavior for your child - that being, people need to make sure they are taking time to refresh or reinvigorate themselves for their own emotional well-being.
Good luck, set an appointment for that haircut, make a date with a friend for a coffee, or just do something for yourself!