You may have over-done it the other day by using the walker at the store. You might want to consider just using the walker at home right now, or in the yard or on your street. Don't push yourself too hard. I know of people with CP who walk unassisted, but at times, even they will still use a wheelchair (for long distances- like amusement parks). I think it's wonderful that you are not giving up hope that you can walk with your walker, and I believe that you can. Just take your time though, don't expect too much of yourself all at once. You don't want to risk injury as that would cause further set backs. Since you didn't mention what type of CP you have, I'm going to assume (sorry!) that you have Spastic Diplegia since it's the most common type. There are lots of ways to reduce spasticity; such a stretching, Botox and Phenol injections, oral baclofen, baclofen pumps, electrode therapy.... You might want to consult with your Neurologist or a Physiatrist (doctor of Physical Medicine) if any of these options are right for you. Do you wear DAFOs? You can even get those to wear at night as well, to give you a continuous stretch. If you do have spasticity, I'm sure you know that swimming in warm water is better for you. It helps relax your muscles more. If you have access to a heated pool, aqua therapy is one of the best forms of therapy for anyone with CP, regardless of type. You might want to check out Hippotherapy as well. It's one of the greatest therapies for improving trunk strength, balance, and coordination.
I wish you the best of luck!
My 4 year old has Spastic Diplegic CP
www.specialparent.org (server problems right now, but save the link- it's a wonderful support group for parents of children with disabilities and special needs!)