how do you convince yourself that it's okay to eat more? help!

if i eat 900 calories in a day, i feel like purging (although i don't). I'm never really happy with myself at the end of the day unless i've had about 750 calories. the thought of eating more, even though i exercise very regularly, just makes me feel so frantic. help!

That's definitely not enough food, and neither is 900 calories a day.

You need to seek help from a medical professional, because if you keep eating that little, it will cause serious harm down the road. 

With a problem of this severity, the only thing that is going to help is a long term, working relationship with the right therapist. 

We can give you all sorts of tips about how to increase your calories if you want.  For instance, get rid of all "diet" foods and eat the regular versions.  You'll be eating the same volume of food but the calories will be more.  Also, stop counting calories and stop logging your food.

wow, i don't think I could possibly eat anything without thinking about how many calories i'm taking in. That's all i've thought about for 2 months straight, and It's completely involuntary now. 

Well, if you're losing weight faster than a 1-1.5 lb/month, then adding 100 calories per day will still make you lose weight. Try 850. You'll still lose weight. Then try 950. And so on until you're up to a safe level.

I find that I can eat more when I DO count everything because I don't have to grossly overestimate to avoid the fear of accidental underestimation. So different approaches work for different people.