You live and learn. At any rate, you live.
— Douglas Adams
One thing that changes (or is supposed to change) in one’s life when one takes seriously the concept of ein od milvado — there is nothing but G-d — is the attitude towards life. When things “happen” to one, and when different events are a part of “life” or “nature” or “world”, then one is allowed to get upset and angry and disappointed. It may not be healthy, but it’s logical to get angry at life being unfair, at “the Universe” tripping you up, at some jerk with a huge shovel attached to his car pushing snow into your driveway (no, this hasn’t happened yet; I am just reliving the happy memories and looking forward to their repetition this year).
On the other hand, if you believe that G-d is creating every single molecule, every single aspect of the world ex nihilo every second, that He is present in every event in your life, then things don’t just “happen” and nothing is “just life”. Everything that you experience and encounter is created by G-d — personally for you. And it’s a little rude to get angry at G-d, to get impatient or disappointed with Him. After all, He knows better and is wise and merciful.
A couple (or triple) caveats. First, this does not deny the freedom of will. We all have the freedom of will. But, someone’s freedom to choose whether to punch me in the nose or not has nothing to do with me getting punched in the nose. I got punched in the nose because G-d wanted to punch me in the nose. At the same time, He gave (for a different reason) someone a choice whether to punch me in the nose or not (as a part of His ongoing relationship with that person). Had the person chosen not to punch me in the nose, G-d would find some other way for me to get my nose smashed. So, don’t get angry at that person for doing any wrong to you. Get angry at G-d — or, rather, don’t!
Second, this does not mean that we should not make our lives better. The whole concept of tikkun olam (and that starts in your own backyard) still applies. Again: you have freedom of will. Just because you bought a hat that’s one size too small doesn’t mean that you have to say “oh well” and accept it. Go to the store and exchange the hat. But, if for some reason you irreversibly lost $1.25, don’t get angry. Since there is nothing you can do about it, G-d wanted this to happen to you, and therefore, it’s for the best. So, l’hatchillo, be proactive. B’dieved, be grateful for your life.
Third (and this is an extension of the second), this does not deny the concept of prayer. Davening really deserves a post (or ten) of its own, but the point is the same as above: we have a freedom to change our lives. When we go to a store to change a hat, we take a physical action. When we daven to become better people — and as a result, become more deserving of better things in life — we take a step in a spiritual direction.
And again, you are not allowed to be depressed about your spiritual state either. As Tanya states, you’re allowed to be bitter — for the purpose of realizing that something’s wrong and deciding to change it. But then, Alter Rebbe says, you must get rid of all negative emotions immediately and again look at the world and yourself with joy. Even though whom you become in the next second depends on you, whom you have become, at this point of your life, was brought about by Hashem. L’hatchillo, be proactive about improving yourself. B’dieved, be grateful for who you are. Otherwise, you’re being ungrateful to Hashem, personally.
All of this is not just some psycho-babble. Living and serving G-d (and for a chossid, these two things are the same, really) with joy is a mark of a Jew who takes derech of Chassidus and the doctrine of ein od milvado seriously. Everyone can accept some philosophical concept in theory. Living with it is more difficult. But we have to. We are expected to.
P.S. Oh yeah. Nivel peh is bad. There. :-P