I have a dark line on mentolabial sulcus. It looks bad and it gives me a feeling of self inferiority. What can I do about it?

I suspect you're referring to the deep depression which gives off a shadow which appears dark. For this, putting in a filler is a nice option. If there is actual pigment there due to melanin, then various fading/bleaching creams that contain such things as: retinol, glycolic acid, azelaic acid, kojic acid, vitamin C, hydroquinone, etc can be used. Your dermatologist will be able to properly guide you once you're assessed in person.

You are probably either seeking help regarding hyperpigmentation of the skin in the sulcus or are referring to a deep labiomental sulcus . Skin hyperpigmentation due inflammation or sun exposure can be dealt with by skin lightening topical creams for about 3 months . Alternatively skin lightening peels can be done in intervals of 2-3 weeks to achieve the desired levels of lightening . Hyaluronic acid fillers and injections may be used to gently fill a deep sulcus thus making the skin appear lighter.please consult a plastic surgeon follow accordingly.

An evaluation of the area and medical history especially for underlying hormonal status would help correctly treat the concern - this area is simple to treat & the results are very gratifying .