Most people are going to have to be constantly vigilant about their diet.
Contrary to popular belief very few people are "naturally thin".
There are several things that are important in my opinion.
1) Having some kind of a regular exercise routine. You don't have to be running marathons but regular exercise is wonderful for your health for many reasons. Personally I'm a fan of lifting heavy but even brisk walking for 30 minutes 3x a week is enough to give you most of the health benefits that exercise offers.
2) Understanding how and why fat gain happens and how fat gain is not the same as weight gain. As pointed out in this thread - did you really gain 5 lbs of fat? Maybe, maybe not. Personally I have found the easiest way to monitor my fat gain it to take a single caliper measurement on a part of my body a couple times a month. It takes literally seconds. This is the one
people should buy if they want to follow this method.
3) Figure out a way of eating you can live with long term that limits your intake but doesn't make you feel too deprived. Phase 4 is one way, but it would never work for me. One popular method is calorie counting. Generally speaking - most of us eat similar things all the time. If you're like this - you may want to simply monitor your caloric intake for a while and see the aproximate range of calories you maintain at. Other popular methods are rule based diet. Example- the white diet. You don't eat anything white. Kind of silly but if you don't eat anything white you'll have a hard time over eating. It takes a little bit of experimentation but my advise is to first consider how you like to eat and see if there is a dietary plan that mimics it.
Well this was a long post but I'm not done yet. There are a number of biological reasons that phase 1 is much harder now.
1) You're out of the routine. It takes a while to adjust and it's much easier to adjust when your body has a lot of fat to draw from.
2) Your leptin levels are much lower now. If you read enough on this forum you'll find that every single dieter who has only a few lbs to lose has trouble with this diet and leptin is a big reason because leptin is a big controller of appetite.
3) Your insulin sensativity is increased. The body prefers carbs for energy and it's very natural for a non insulin resistant person to feel more energy when they drop the carbs down.
Realistically a day or two of phase 1 is something most people who have only a few lbs to lose is pretty easy because it takes a couple days for your glycogen levels to drop down to the point where you'll start feeling lethargic. Obviously everyone is different but based on the way you're feeling you can certainly grit down and embrace the suck or you can do something a bit more moderate. Another popular option for people trying to get off those few remaining lbs is EOD dieting. PSMF (phase 1) one day and maintinence calories the next. It's slower but much more bearable.
Alrighty - longest post ever is done.