I was recently baptized for a second time because I felt that my first baptism was invalid because I wasn't truly converted and turned back to my old ways too quickly. But with this baptism I feel that I didn't repent before I was baptized because I went back to my old ways the next day. I have one sin I struggle with really bad, and have for years. It's one that I won't publicly announce or talk about but it's something that I went back to again quickly. My question is: do I need to be baptized yet again? I am a member of the church of Christ.
Repentance cannot be determined by how soon you are trapped by sin again. It is possible to repent and fall back into sin on the same day. "And if he sins against you seven times in a day, and seven times in a day returns to you, saying, 'I repent,' you shall forgive him" (Luke 17:4). Repeated sin does demonstrate a problem, but it doesn't necessarily mean that your baptism was invalid.
Baptism is the entering into a covenant with Christ. It is similar to being circumcised under the Old Testament. "In Him you were also circumcised with the circumcision made without hands, by putting off the body of the sins of the flesh, by the circumcision of Christ, buried with Him in baptism, in which you also were raised with Him through faith in the working of God, who raised Him from the dead" (Colossians 2:11-12). Therefore, if you truly were not baptized then you would not be a member of Christ's church either.
Along with entering into Christ's kingdom, God also forgives us of our sins. "Or do you not know that as many of us as were baptized into Christ Jesus were baptized into His death? Therefore we were buried with Him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even so we also should walk in newness of life. For if we have been united together in the likeness of His death, certainly we also shall be in the likeness of His resurrection, knowing this, that our old man was crucified with Him, that the body of sin might be done away with, that we should no longer be slaves of sin. For he who has died has been freed from sin" (Romans 6:3-7). Baptism does not provide immunity from sin. But at the same time, baptism does not excuse continued sin either. Paul brought up this topic because people were continuing their sins after baptism. "What shall we say then? Shall we continue in sin that grace may abound? Certainly not! How shall we who died to sin live any longer in it?" (Romans 6:1-2). Paul did not argue that their baptism was invalid or that they needed to be baptized again. He told these people they needed to stop sinning.
In a sense, you are distracting yourself from the real problem, which is dealing with your weakness for returning to your sins. A Christian can obtain forgiveness of sin by admitting his faults to God. "If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness" (I John 1:9). But it also means we cannot accept sin in our lives.
Not knowing what sin your are struggling with, I can't give you any suggestions for dealing with it. However, you need to realize that often dealing with sin by ourselves is not easy. That is why we are told, "Confess your trespasses to one another, and pray for one another, that you may be healed. The effective, fervent prayer of a righteous man avails much" (James 5:16). Don't let pride get in your way of reaching out for help.