Weight loss & Exercising [Struggling to meet BMR]

Background information:

Age: 21

Weight: 91 Kg

Height: 161-163

For a long time I've been eating rather little due to a large lack of appetite; this has been going on for a little over two years. I can maybe stomach a meal or two a day. I am well aware this is not healthy and have tried to incorporate more food into my diet by trying to train my body to eat even when I'm not consciously hungry. I continuously gain weight despite this. Over the last year and a half I have gone from roughly 63-64 kilos to 91 because of lack of appetite.

Now I typically eat...

Breakfast:

Milo cereal with low fat milk

A banana

Lunch:

Two slices of bread with tuna on them.

Snack:

Apple

Dinner:

Usually chicken with salad.

I have been diagnosed with Ankylosing Spondylitis. I cannot sit anymore for no more than 20-30 minutes at a time. I must keep active to keep mobile, reduce inflammation and for my sanity. My ESR is 44 (Should be <10) and my CRP is 17 (Should be <2.9) despite being on NSAID medication 

I am currently on Endep for migraines and Meloxicam for the inflammation. I have a membership at the local leisure centre that includes unlimited access to group exercises, the sauna, the spa, the steam room, the pools and the gym. I can't do high impact activities and so I swim for roughly 60-120 minutes a day ( I am still a novice and still learning to swim, so they are often not set laps! Leisure swimming, maybe some water aerobic classes). I have found that this has helped my pain levels significantly and it is not something I want to compromise on. I will also be starting my gym exercises on Monday when my gym plan is organised by my instructor.

In order to keep my inflammation down (Thereby slowing down the progression of AS) and to keep a level of mobility I have to exercise daily. However, because I've gone so long without eating properly I have to now eat a large amount (1600-1800 calories) with my exercising(According to the site) in order to lose weight and be healthy. I am severely struggling to eat this amount of food. I am looking for tips on small things, maybe nuts or whatever that is both healthy, beneficial and high in calories so that I am able to safely meet my weight loss goal. As a full time student living away from home, my parents help a little for my membership, but this means that I can't spend a lot of money on expensive food. I do not have a sweet toothe. I do not eat chocolate nor do I drink soda. I drink orange juice, milk and water primarily. 

Any tips would be very much appreciative! My main worry is that I won't be able to eat enough to lose weight with the amount of exercising I am doing, as this level of exercise will only increase and I absolutely cannot limit my exercising. My plan is to lose inches/weight so that I'm around the 60 kg mark again.

Thank you very much! 

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