calorie intake

Well you're certainly not eating enough looking at the last few days! No wonder you have no endurance for your workouts.

MFP already puts you at a deficit so you should be eating the 1.300 odd that it has you set at not the 300-500 you eat most days!

When you exercise if you use a heart rate monitor then you can eat all your exercise calories back as it's an accurate measure, if you use MFP's estimations then I'd eat up to 50% back as it often over estimates what you burn.

Seriously though you're really not eating enough.

You have to burn more than you eat to lose weight. But it has to be a healthy deficit - too high and you'll see negative side effects (even though the scale may read smaller numbers).

On a related note, the deficit is all that is necessary to lose weight (assuming no heath issues). You can reach a deficit with no exercise at all, or you can get there with a combination of diet and exercise. Exercise has other benefits as well, but it's not required for weight loss.

To figure out what your daily intake should be, either use MFP (but understand how it works with calorie goals, exercise, and weight loss goals), or use one of hte TDEE estimators (google them) and set your daily goal to something like TDEE - 10%.