Thank you for you question!
In your question you do not specify in which way you are “failing” in purity. So, I will give a general assumption to your question.
I will assume that it is sexuality or something that deals with it.
1) You may be engaged in sexual activities
2) You may be having sexual dreams which will result in nocturnal discharges
3) You may be indulging in onanism
4) You may be attacked by “impure thoughts.”
For #1 through #4, practicing the code of conduct is mandatory. That is amrit vela, daily murli class, satvic food cooked by yourself (no onions or garlic and avoid hot-chilly items as well,) celibacy (stopping any behavior that could drive you into this, especially if you have not gained inner control yet.), daily meditation, keeping your chart. Those are the pillars and must be followed; otherwise, you will be suffering for not being able to fully follow this Brahmin path.
For this you need to be honest with yourself. It is your “choice,” after all. If you have the determination to explore and conquer sex-lust, you will be facing many obstacles; but it can be done especially, if you are a Brahmin soul.
For #1 above then, I recommend for you to be away from any “temptations,” until you grow stronger. This takes time, but you will know the right time.
For#2 above, that depends on many factors. Age is one of them. If you are in your 20’ or 30’s and you have been sexually active in any way before, those episodes will occur. However, they will diminish gradually as your body becomes older AND you are following the code of conduct.
Patience is needed. You may lose a battle but no the war.
For#3, that needs to stop altogether. A way to do that is by opening your feelings, and here is when the experience of yoga will help. Hopefully you have experienced Baba and felt that in your heart. Also, to be in Nature and to be taken by its beauty, to be sensitive to it, will be of great help while keeping your urges (just a sanskara) down. Determination is needed along with feelings. Work on the heart chakra and on balancing your male/female energies. If you are a male; by doing some activities which are typically given to females and if you are a female, the opposite.
For#4, those are part of the tests. You will have plenty of those while in the first stages of practicing celibacy. Just keep unmoved and continue on despite those thoughts. Once your heart chakra is open (find more information on that) things will start changing in your life and the quality of your celibacy as well.
Sex-lust is the inability to express through the energies of the heart. At that point, the first 3 chakras is all what is available for us and sex is the energy which we will not able to transform into good wishes and pure feelings but rather that energy will be wasted for we are unable to direct it nor control it, when we only are aware of the 3 first chakras, which is how the majority of the people function.
Finally, you will need lots of practice with meditation/yoga and to have lovefull yoga with God. That is how your heart chakra will open, the Brahmin way. I would strongly recommend this experience to counteract the corporeal feelings of sex lust.
Keep your mind busy. Be involved in worthwhile things.
This is a new life style.
If you are firm on this and you are no slacking off but giving it 100%, there is no need to feel guilty. that is just waste at that point. As I said before, the battle maybe lost but no the war. Just keep that in mind, and despite the setbacks continue on while looking at your chart to see what may have caused the issue. Check and Change. Experiment. Try new things.
You are experimenting with yourself. You are experiencing new ways.
You will not give up, if this is your call in life.