When it comes to weight gain you need to realise that a few kilo really won't make a difference to your body. You might think, clothes fit me now, I feel ok, I will be huge when I gain a few more kilo. But you know what 2-5kg really doesn't make so much difference to your body. It can be hard to accept or believe, I mean the weight has to go somewhere.... but it's not like the weight just turns to fat and sits on your stomach like so many are worried about. It redistributes around the body and so it's not always so noticeable. Just like losing 2-5kg isn't always super noticeable unless you are already very slim.
Also remember that a normal persons weight fluctuates 2-3kg throughout the day and that is just due to water and food weight.
Weight gain might seem like the worst thing ever but it really isn't. Think rationally what will a few kilo of weight gain really do? It won't make any difference or no drastic difference to your body. And think like this. ... if you think those kilo make a huge difference and you can't stand the way you look then you can always lose the weight again. I don't advise that, but I mean it's the truth....
The first thing I would suggest is to try to gain the weight and then take it from there. Know that the last few kilo can make a big difference in your mental health. But also ask yourself if you are scared of gaining a few kilo are you really letting go of your eating disorder? Letting go means letting go of weight and control. ... allowing your body to be healthy at the weight it settles at.
Weight gain is not a bad thing, infact if you need to gain weight it's necessary and super important. If you have been told that you need to gain weight then listen to that advice. And even if you have calculated a healthy weight yourself, don't give up in the end.
Your body won't change from a few kilo extra, but your mental health can improve.... you need to learn to let go and to trust. Trust your body and trust your doctors!