Phenibut can help lots of serious issues, but I personally wouldn't take it unless I talked to a doctor first. Currently there is a lack of information in literature about physiological dependence and addiction to the substance, yet I've read plenty of people on forums describing having trouble with it.
This case report by Brunner and Levy (Journal of Addiction Medicine. 11(3):239–240, MAY 2017) shows a case of someone who became addicted and who struggled to get off of it even with formal chemical dependence therapy visits to his house, and eventually needed phenobarbital to get off of it. Although that it may not be that extreme for everyone, it would be better to stay safe, as such physicians are calling for more study to determine if Phenibut should be a controlled substance.
I would not use it for anxiety, libido or sleep problems. You can easily develop a crippling dependence on it so it is too dangerous to use for any of those things. It's sort of like benzodiazepines in that sense and it has hellish withdrawal symptoms.
Phenibut is unpredictable and unreliable. There are no established doses or schedules of taking it, only urban myths and legends. It's one of those things that everyone tries and nobody stays on it for the unpredictable lack of effectiveness.
There are actual safe and effective drugs for sleep - zolpidem (Ambien). For libido, there are no established drugs at all, because it is such a complex neurochemical process depending on gazillion factors. You could try "tonic" type herbs like red ginseng but they may exacerbate irritability/anxiety.