Thinning hair out to certain age is good but after 18 years you should not thin your hair. Your thick hair determines the healthy nature of your hair. Let it grow healthy. If this thickness
irritates you then you can go for an straigthening procedure in which you will feel good.
You should see a hairdresser to thin out your hair. There are products that can do this, but they can cause too much damage. A hairdresser will can cut your hair and thin it out without damaging it.
I would definitely consult with a hairstylist, mine recommended a shoulder length inverted bob, and though the back is so much lighter, I did not have to sacrifice length.
Use thinning scissors, most salons have them. This way you'll reduce volume and change your overall hair texture. They're not meant for cutting, just styling.