Charlotte, gaining weight is not that hard, just stop exercising and start eating lots of junk and you'll gain weight. Gaining muscle however is another thing.
Basically, 1 pound equals aprox. 3,500 calories. You have to find a way to manage your diet and the amount of calories you burn weekly so that you end up having a net gain of 3,500 calories at the end of the week to gain 1 pound. The calories have to come from lean protein sources and complex carbs eaten together, 5-6 meals every 3 hrs or so (I think you already do this.) The amount of protein can go from 0.5g per pound of weight to 1.2g per pound of weight in women. Bodybuilders usually eat 1-1.5g per pound of weight when they're preparing for competitions, but I think going up to 1.5g per pound can be dangerous to the kidneys if it's done on a daily basis.
When it comes to gaining weight from lean muscle it is better to stop doing total body weight workouts and start concentrating in lifting weights for certain body parts 1-2 times a week (better if you do one big muscle group with a smaller one for example chest with triceps, back with biceps, etc.) You have to lift as heavy as you can handle and try to increase your weight every 2-3 weeks when possible or sooner if the exercises feel too light. Cardio should be limited to 3 sessions of 20-30 mins a week, but those sessions should be done after the weight workouts and should be moderate to high intensity (you don't want to lose your cardiovascular capacity because you're doing mostly weights.)
Remember also that due to our hormones, it is harder for us women to gain lean muscle than it is for men. The amount of testosterone in our bodies is minimal compared to men, therefore it is easier for a man to follow the regimen mentioned above and start gaining pounds of muscle in a matter of months. Women have little testosterone but lots of estrogen and estrogen makes it really hard for us to gain lean muscle. Gaining lean muscle the "natural" way is really hard for women, but it is achievable in a long period of time (for some women it can take months and for others years; genetics and age play a role, and of course conditions like hyperthyroidism are not helpful either.) I hope this helps.
Want to bicep curl my son? He's a good 24ish pounds...lol