I’m a recovered bulimic, and I cannot fast. I tried when I first converted, and found myself back in dangerous territory. You probably know what I mean - the rush, the buzz, the sense of purity… I quickly went and broke my fast with a normal meal and prayed on it. I think I even posted here because I was so troubled by my near-instant response. The entire experience scared me witless.
Now, a few years later, it’s easier for me. I don’t fast on any of the fasting days. I go to Mass, and I keep a quiet day - no music generally (I can’t turn it off when I’m in a public place!), no television, prayer & reflection. I eat simple meals but not overly bland. I do try to avoid meat but sometimes I goof because I try to keep my eating “regular.”
Just as I’ve learned my triggers, and I avoid them! - I also have learned to recognize my limits, and to honor them. I honor the Lord by honoring my limits. Eating disorders are as much of an illness as diabetes and hypoglycemia - and you should treat your ED as such. Once your are out of treatment, before Lent, come up with your plan for days of fasting & repentance. Perhaps using the day for a quiet personal retreat, or attending Mass, or praying the Rosary would be a useful and special alternative.
I think the key point is to treat fasting days as solemn and reflective days to honor God. By making them special and out of the ordinary, we remember to take the time to think outside of our daily lives and put God first.
My prayers of support and healing for you during your time of recovery. There is light on the other side. You should be very proud for the hard work you are doing, of the fact that by getting help and healing you are truly honoring God’s gift of life.