I guess there are different schools of thought about weight-loss; and I have never heard the one you are talking about. It is important to plan just as you would for anything else, but obsessing is another matter. Yes, we can overdo anything; you need to find a balance.
Just like you said, you cannot continue to eat too much and expect GOD to take away the consequences of that. We do have some responsibility for what we put into our mouths -- but we can pray for the HOLY SPIRIT to give us the power to make better choices and be more self-disciplined.
So, do the basic planning; buy the right foods; and follow your plan the best you can (with the Holy Spirit's help). If you make a mistake, forgive yourself, and continue on. I think this WL process is very similar to our Christian walk, as I see so many parallels between them.
We need to read the Word, follow the LORD's commandments to the best of our ability, asking the Holy Spirit for the power to do the best we can; and rely on GOD'S Grace for the rest. As time goes on we grow and mature in our walk (becuz practice helps us to do better).
As in the Christian walk, you will be tempted and tested in your WL journey. JESUS will forgive you for the mistakes; and you must forgive yourself too (see Colossians 3:13). That's why you cry, you know -- becuz you're not. You are putting too much pressure on yourself. What you need to surrender is your "Expectation of Perfection". None of us are perfect; none of us can be in this life. That will happen in the next one. Your vanity may be a reflection of this; I mean, it may a veiled desire for perfection. Remember -- there is a difference between being presentable (clean & neat) and vain; the first is OK, the second one can present issues.
In our Christian life, what we need to surrender is control -- we must surrender to GOD and put our TRUST in Him for guidance and direction; plus the power to overcome any obstacles we may face, including our flaws and imperfections (shortcomings). That includes your weight-loss journey (and vanity taking GOD's place in your life).
Also, I would ask GOD to reveal to you WHY you are eating the wrong things -- to reveal to you if you are eating out of boredom, just becuz the food is there, or as an emotional response to stress, or whatever (these are my most common reasons for eating things I shouldn't). Plus ask Him for the wisdom to know how to manage those issues better. I think that GOD would rather that we rely on HIM for power, strength, and comfort instead of the food. Excess food isn't healthy for us; but an extra serving of GOD can only be an advantage, in my books.
I hope this helps you in some way. Don't give up; but continue on no matter what (BTW, the Christian equivalent is FAITHFULNESS) ...