Can I focus too much on myself for assurance of salvation?
Yes, you can. You can become fixated on your self, which will not produce the very thing you're trying to produce. The person who is analyzing their motives all the time, analyzing their fervor all the time—"Am I lukewarm and going to be spit out of his mouth?"—or analyzing their doctrinal clarity all the time will defeat the very thing they're after.
The reason is because we don't get assurance mainly from looking in the mirror. We get assurance mainly from looking at Christ and the sufficiency of his work on the cross.
That's not to say that we don't do spiritual introspection, because 2 Corinthians 13:5 says to test yourself to see if you are in the faith. And the whole book of 1 John is written to give tests of faith, such as "If you hate your brother you don't know God"—things like that.
But once you've done the introspection, seen what there is to see, repented the best you can of what's sinful, and taken heart from the little evidences of grace, what do you do next? You don't get assurance from that glimpse mainly. Do what you need to do there. Do your spiritual business, and then you get your eyes focused on the hope-giving promises.
It says in Romans 15:4, "Everything that was written beforehand was written for our instruction, that by the steadfastness and encouragement of the Scriptures we may have hope." So you get your longing for hope fulfilled by being rooted in the Scriptures, especially in the center of the Scriptures, which is the cross of our Lord Jesus Christ. By looking at Jesus we are awakened to feel the sufficiency of his work for sinners.
"Beholding the glory of the Lord, we are being changed from one degree of glory to the next," Paul says in 2 Corinthians 3:18. And we see the glory of the Lord, as he says five verses later, in the knowledge of the gospel of the glory of God in the face of Christ.
So fixing our minds and our hearts mainly on the center of the Bible—which is Christ crucified, risen, reigning, and providing an alien righteousness that we have by faith alone—this glorious center of the gospel is where hope is awakened, preserved, kindled, and assurance is maintained.
I deal with a lot of people who, when I lead them through steps toward assurance, will come back at me with, "But this, but this, but this," etc., always saying that they can think of another reason why they shouldn't have assurance. And you know, ultimately you get to Christ, to the gospel, to the cross, and there is nothing more you can say. You say, "Go home after this service and simply meditate on the cross."
I once heard a woman who had come out of the Jehovah's Witnesses give a testimony, saying that she was about to commit suicide because she just couldn't do enough works to give herself the assurance that she felt she had to have (which she thought could only be by works). But before attempting suicide—it was on a Good Friday evening—somebody had told her to read through the Gospel of Luke.
She knelt down at her bed, and she began to read through the whole Gospel of Luke—this was pre-suicidal, to see if there was any last hope. And when she got to the final chapters, it was the beholding of Christ on the cross forgiving a thief and saying, "Father forgive them, they do not know what they're doing," that broke her. And she gave a testimony the next Easter morning to the effect that it was looking at Christ crucified that rescued her from a suicidal lack of assurance. All of her introspection and performances did not do what beholding Christ could do.