I find myself in constant doubt. I try to do as much as I can for the Lord with a heart and mind centered on the glory of God, but I question my own motives. I want to do the things that God intends but I feel that my own self-doubt is holding me back from having a whole heart for the Lord’s will. How can I maintain a Godly mindset while fulfilling what he intends of me? Is it simply a self-doubting mindset? If so, how can I remind myself that what I do is for the Lord?
I have included this whole long question because it so clearly reflects a very common problem that I suspect most of us have. In fact, I suspect that those who do not at least occasionally have this problem are going on without sufficient self-reflection. But, while a certain amount of reflection is important, it must be balanced by a firm knowledge of God’s superintending love for us. So, let’s begin there.
Our security, not only for salvation (Jn 10:27-30) but also for service is certain. We are accepted, not just in God, but in the Beloved One. (Eph 1:6) Christ has brought us into a place where we are in God’s affections even as Christ is. We need to learn to rest in that fact. We are created for good works. (Eph 2:10) And, God Himself works in us both to will and to do his good pleasure. (Phil 2:13)
Self-occupation is a great hindrance to service even as you admitted in your question. We naturally have questions: “Am I doing enough?”, “Am I doing the right thing?” The Preacher admonishes “Whatever your hand finds to do, do it with your might” (Eccl 9:10). If our hands are available, God, who loves to bless us, will fill them. But, be warned it might not be some great work that would pander to our pride. The apostle writes, “So, whether you eat or drink, or whatever you do, do all to the glory of God.” (1 Cor 10:31) The simplest tasks of life should be done to glorify God. (See Mt 25:21)
Yet, in a world which constantly draws our attention to its glamour, we need a warning. A familiar hymn goes
“Have I an object here below, which would divide my heart from thee?”
Certainly, the world can draw our hearts away from simple devotion to Christ. The world in a religious cloak is still the world. Even “Service” could be such an object. But, if our heart is delighting in Christ in simplicity, we will find true service. One more thought, however. We may not always know the way He leads until after the journey. This frees us further from self-occupation. Our following Christ must be in the simplicity of faith.
 A book which gives good lessons of this type is The Golden Thread by Elsie Koll. Unfortunately, it is now out of print. Mrs. Koll was a missionary to China and describes in this interesting book many instances when she could look back, sometimes after several years, to see how the Lord led her when she had had to make a decision about what to do.