I see in the Scriptures that spiritual battles are spoken of, especially by Paul. What are spiritual battles? Would it constitute a strong temptation Satan places to go back to an old sinful habit?
I feel sometimes that my strong temptations toward sexual things and lust make me an impure person, who is unholy, and I have the feeling that no other brethren (mainly men) struggle with it as I do. It's like when my spiritual life is going well, and I'm doing good to others, I'm praising God. Then a thought comes to my mind: "This is too good to be true. You aren't good or moral enough to live an upright life." Then I dwell on the thought, believe it, and end up sinning. It's almost like I think: "Who are you kidding? You're a sinful and immoral person." Then I think, "After this sin, God will not accept me back, and I will miss heaven." I get to feeling hopeless and helpless. I desire so much to be morally excellent in God's eyes all the time, but Satan or my own self is keeping me down.
Can you please help?
If you think about earthly battles, you can view them from the individual soldier's point of view or look at them from the struggle between the various sides.
Paul's description of the armor of God is from the soldier's point of view: "Finally, my brethren, be strong in the Lord and in the power of His might. Put on the whole armor of God, that you may be able to stand against the wiles of the devil. For we do not wrestle against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this age, against spiritual hosts of wickedness in the heavenly places. Therefore take up the whole armor of God, that you may be able to withstand in the evil day, and having done all, to stand" (Ephesians 6:10-13). As a soldier, life can be tough and you can't always do as you want. "You therefore must endure hardship as a good soldier of Jesus Christ. No one engaged in warfare entangles himself with the affairs of this life, that he may please him who enlisted him as a soldier" (II Timothy 2:3-4).
But the war we fight in is not ours alone. We are a part of a large army. "For though we walk in the flesh, we do not war according to the flesh. For the weapons of our warfare are not carnal but mighty in God for pulling down strongholds, casting down arguments and every high thing that exalts itself against the knowledge of God, bringing every thought into captivity to the obedience of Christ" (II Corinthians 10:3-5). When you see yourself battling alone, you've lost sight of the bigger picture. "No temptation has overtaken you except such as is common to man" (II Corinthians 10:13). You aren't the only one battling. In fact, if you see yourself as being alone, you've gotten separated from your fellow soldiers and the support that they can lend you. "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). While you have to do battle, there are many standing with you to lend you support. "Bear one another's burdens, and so fulfill the law of Christ" (Galatians 6:2).
Not all of the enemy's attacks are obvious. There is a reason Paul warns, "Therefore let him who thinks he stands take heed lest he fall" (I Corinthians 10:12). You describe one trap well: the planting of doubt that eventually causes you to drop your shield. Yes, all of us sin but that doesn't make you a sinful person. Take a look at "Saint or Sinner?" A sinner stays in sin. Being righteous is not a priority to him. But a righteous man keeps battling sin. "For a righteous man may fall seven times and rise again, but the wicked shall fall by calamity" (Proverbs 24:16). Take a look at Psalms 51 (A Psalm of Penitence). David wrote this after being confronted about his sin with Bathsheba. Notice that he didn't want to spend time with his guilt but rather to get back on track with God.
Being a Christian is not about being perfect. It is about not giving up the fight.