just had a doctor tell me i'm a stone underweight

And i am so worried that i am going to just let go my self-control, binge, slacken off excercising, and end up putting on about 2 or 3 stone really quickly.

for example, usually do 30mins running about 4-5 times a week, and i now haven't been running for 2 days and have eaten a bit more and feel TERRIBLE, even though i know i need to put some weight on. I suppose it took such an effort to lose all this weight (i've lost about 5 stone) that i would feel like i've failed myself if i put on any again.

Is anyone else feeling the same way?

Unless there's a medical reason to exercise less or you have too little body fat, don't cut your activity levels. Just eat more. That way you gain more lean mass and less fat mass.