OK, simple question: I want to know what I can do to just serve God and know it's Him I'm serving.
"Let us hear the conclusion of the whole matter: Fear God and keep His commandments, for this is man's all. For God will bring every work into judgment, Including every secret thing, whether good or evil" (Ecclesiastes 12:13-14).
Many people try to determine God's will based on their own desires or feelings. Of course, what we want and what we feel changes. What you want and feel is different from what I want and feel. This isn't the way of God but of chaos. The book of Judges in the Bible talks about a period of Israel's history where everyone went from bad to worse. They constantly left God and when they got into trouble begged God to rescue them, only to turn away from God once they were safe again. The last verse tells us why: "In those days there was no king in Israel; everyone did what was right in his own eyes" (Judges 21:25). I suspect that this is what you've been trying to do and it isn't working out for you.
The matter isn't difficult. God didn't leave us to wonder how to serve Him. "Now by this we know that we know Him, if we keep His commandments. He who says, "I know Him," and does not keep His commandments, is a liar, and the truth is not in him. But whoever keeps His word, truly the love of God is perfected in him. By this we know that we are in Him. He who says he abides in Him ought himself also to walk just as He walked" (I John 2:3-6). You know you are serving God when you look at God's teachings and see that you are obeying Him.
It is much the same with faith. Faith isn't just a declaration of "I believe in God!" Faith is based on knowing what you believe in. "So then faith comes by hearing, and hearing by the word of God" (Romans 10:17). If you don't know what God has said, then you can't truly claim to believe in God.