Me and my girlfriend keep falling into lust

Well the title is simply what this post is about. Me and my girlfriend keep falling into lust many of times and we don’t know how to stop. We’ve prayed, repented, and have tried to have self control but every time we fail. We are planning to get married eventually but until then I don’t know what else to do. I feel like such a failure and that one of these times God’s grace will no longer cover me because I keep disobeying him. I understand God forgives but when sin becomes habitual like this, can’t God’s grace stop covering us because we have knowledge of the truth but still choose otherwise? I don’t know, I just need clarity in all this chaos honestly.