I am 18. I got saved a few months ago. Although I love God, I am constantly messing up. I have a stumbling block for lusting homosexual thoughts and laziness to name a few problems. I know what I am doing is wrong, and I keep confessing, but I still mess up. I am slowly giving into despair. I am tired of it. I’m no longer sure if I am still saved or not. What must I do? I’ve done everything, even going to the altar. What’s left?
Since you mentioned "going to the altar," which is not a concept found in the New Testament, and since I don't know what you did to get saved, I can't comment on whether you did all that you needed to do or not. However, if you would like to read "How to Become a Christian," you can look at the verses mentioned there and determine if what you have done matches what is found in the New Testament.
Many people think that all the work is in becoming a Christian, not realizing that there must be a continual effort. "Be sober, be vigilant; because your adversary the devil walks about like a roaring lion, seeking whom he may devour. Resist him, steadfast in the faith, knowing that the same sufferings are experienced by your brotherhood in the world" (I Peter 5:8-9).
No matter what type of thoughts, lusting is sinful. This is because what we accept and strongly desire in our minds is what we eventually end up doing. "For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, thefts, covetousness, wickedness, deceit, lewdness, an evil eye, blasphemy, pride, foolishness. All these evil things come from within and defile a man" (Mark 7:21-23). Since I don't know what is triggering this struggle, I can't address how to deal with it.
In regards to laziness, I assume you put off things beyond the last minute. Since you recognize the problem, do something about it. Categorize the things in your life: the things you need to do and the things you have fun doing. Do some of the things you need to do first, and then reward yourself with an hour or two doing some of the things you enjoy doing. Knowing that you have something enjoyable waiting for you will make the necessary work easier to finish.
The big thing is not to give up. "For we have become partakers of Christ if we hold the beginning of our confidence steadfast to the end" (Hebrews 13:14).