"I have been in the process of rediscovering Judaism. I am struggling with how to decide what to follow. Whether or not to keep Kosher, and how to observe Shabbat, are important pieces of my struggle. How do you justify not doing some and doing others?"
You are not alone. Many Jews are returning to their religion, and are also struggling with the elements of Judaism. Please note that the name for us, ISRAEL, means those who struggle, who wrestle with God. So not only are you in good company, you are being true to your heritage by wrestling, by struggling with an answer and a response to God's call. One could argue, by the way, that those who do not struggle with all of this may be too comfortable.
At any rate, let me tell you a few things that I think will help you. The first is my all-time number one favorite Chassidic story. As you may know, the Chassidim are the ultra-orthodox, meaning that they are strict in their observance. Keep that in mind as you read and think about this story:
The students of the Rebbe (the number one head rabbi) wanted to see if they could stump him one day, and so the boldest student came to him and asked the Rebbe if he could answer a question. The rebbe of course agreed, and so the bold student asked, "Rebbe, there is a ladder, there are 613 rungs on this ladder corresponding to the 613 commandments. There is someone near the top and someone near the bottom (meaning that the one near the top does almost all of the commandments, while the one near the bottom does few of the commandments). The question, Rebbe, is "who in the eyes of God is higher?"
The Rebbe said nothing. You see, if the Rebbe said the one at the top, the obvious answer, it would have denied the heights, however low, achieved by the one at the bottom, but if the Rebbe had said the one at the bottom, it would have denied the heights reached by the one at the top. And so the students thought they had him! So the boldest student pressed him, and asked, "What is wrong Rebbe, have you an answer?" The Rebbe replied, "I am waiting for the rest of the details of the problem."
The student said, "But we have left out nothing! There is a ladder, with 613 rungs, someone is near the top, someone is near the bottom, who in the eyes of God is higher?"
To which the Rebbe replied, "But you have not told me which of the two on the ladder is moving upward."
It is not where we are on the ladder, it is that we know we have this ladder, truly a divinely given opportunity to elevate ourselves towards God and to be spiritually uplifted through the commandments, and that we are slowly taking steps upward, as you are doing by trying out for a while doing just one or two of the commandments, and then, after you feel comfortable at this new height on the ladder, only then moving to a higher rung.
The next thing I would like to tell you is that each and every commandment has its own merit, its own reward and its own effect on the one who does it. Do not worry what you are not doing, or how many you are doing, just know in your heart that you are wrestling with the idea of even having this ladder, and that you are slowly, cautiously raising yourself up it. Do not leap to a high place, if you lose your balance you will fall off and wind up hating the ladder (hating Judaism), but rather take a small step, get your balance, feel comfortable, then move to another step, try it out. And do not feel badly if it means nothing to you, there are a lot of steps and you will find your place on that ladder!!