The Vilna Goan says something that's very important to know, especially for those struggling with powerful desires. He writes that what a person says he wants, and even what he feels he wants, has no relation to what he actually wants. The Yetzer Hara was given permission by Hashem to make a person feel that they want something they really don't.
The Vilna Goan also says that the same idea applies to what a person feels they can or can't do. The reality has no relation to what the person thinks or even feels that he can or can't achieve. [This is because the reality belongs to G-d. Our own estimates of our strengths and abilities don't affect G-d's ability].
So never give in to what the Yetzer Hara makes you believe you want, because you really don't want it. And never say "I can't achieve sexual purity" even if it looks so hard. For nothing is hard for G-d. So give your heart over to him and he'll do the hard work for you.