Sticking to your guns

How do you do it? How do you stick to your guns after that initial fervor to change a habit? No, really, I am asking how you do it because I seem to fail at this a lot. Here is my for instance... I am working on this journey to better health. I have embarked, taken positive steps in the right direction and yet, I keep goofing up.
Week one, I was at the gym for all three classes and did some walking besides that on my own. Last week, I made two of the classes and failed to do any of the extra work I needed to be doing. Three weeks in and I have gained back the 3 pounds that I lost in the first week. In my book, that counts as a FAIL.
Add to this the fact that I am trying to eat healthier foods in smaller portions and make sure that I drink a lot of water each day. In the first couple of weeks, I was doing okay with this or at least making progress. In the last week, I have been to buffets and bought and ate a whole pack of cookies. COOKIES!?! Um.. FAIL.
So, okay, I guess I am kinda begging for helpful advice or at the very least prayer. This journey that I am undertaking is almost as important to me as my journey to draw closer to Christ every day. If I don't lose weight, my joints and heart will begin to suffer. Not only that, but what single woman in her right mind would be interested in a guy who doesn't care about his weight and health? I would dearly love to be a husband and father one of these years, Lord willing, but doing what I need to do to get there is proving difficult. I am getting in my own way.
I could use the excuses that I was out with friends and that is just where we ended up or that healthier food costs a little more, but I know that the fault lies in me. There is some part of me that needs to be reconciled to a new way of thinking and looking at life. A new way of looking at exercise and a new way of approaching food. This is honestly becoming a real challenge for me. If you have ideas that I can employ on my limited budget, I am all ears. If you too are out of ideas, then just pray for me. Regardless of how hard this journey seems to be, I will walk on.

Blessings,
Stan