I used to thin my hair (just underneath like another poster mentioned she does) but now I just have it layered. It takes out a lot of the bulk and weight and makes it much easier to detangle. It looks a lot healthier too. I'm not sure of your mom's reasoning for not wanting you to cut it but if it has to do with keeping your length, you can layer quite a bit off without actually touching your length. The last time I layered mine I did it by flipping it over my head and had enough to donate to Locks of Love! If your mom approves, you can google how to do this, some people here use Creaclips or banana clips.

Good luck.
CG since 3/1/13
3B; Low porosity, Coarse, & Extremely Protein Sensitive
Very high density, waist length

Co wash: Suave Clarifying or Sun Ripened Strawberry
RO: Sun Ripened Strawberry, TN, or TN w/ veg glycerin
Sealer: apricot oil
Styler: 4 Naturals Spring Back Custard, Pantene Curl Defining Mousse, or BB "Mudd"
Using a hard water filter, microfiber towel, & silk head scarves & pillowcases

Avoiding: protein, coconut, & parabens