Hey, you're "back on the wagon."
It might be helpful to try and remember what derailed you before (or what combination of things). Knowledge is power, and self-knowledge can be a help when it comes to troubleshooting. Changing your lifestyle means finding new solutions, and that's easier if you identify problems.
A few totally random theories - these are problems and solutions that may not apply to you at all - most of them maybe won't - I'm just throwing them out for brainstorming purposes.
1. Did you hit a plateau and get discouraged? (Don't pay much too attention to the scale/pants size this time, set fitness goals as well.)
2. Did you have a lot of pressures on your time that made it hard to stay organized and find the time to prepare healthy food and to track? (Tweak your lifestyle to get max. health for min. time investment - efficient shopping & prep routine, a repertoire of quick, healthy meals etc. - and/or to find more time for your health. Identify the trigger situations when you're pushed for time and short of healthy options.)
3. Did you have emotional stresses (family, work, coping with a long, dark winter) that led you back into your old ways, or did you find your new healthy diet stressful because it didn't satisfy you? (Work on finding substitutions for unhealthy things you like to eat, and new healthy options that you really like, and non-eating-related ways to be more relaxed or happier.)
4. Did you have some form of physical stress, like sleep problems, illness, side effects from medication? Was your diet stressing your body (not enough calories, not enough of all nutrients?) (Be gentle with yourself, listen to your body, and take things at your own pace.)