How To Relax The Forehead Muscle?

After years of squinting in the sun, my forehead muscles in between my eyebrows and above my nose area have permanently tensed so that I have a very serious and angry look. I tried botox twice to relax the muscle and it worked for 2 days and then tensed up again. I tried rubbing anti inflammatory gels on my forehead to relax the muscle but to no avail. Is there anything that can be done to relax the muscle ?