MasterDoig, not the answer you want to hear, but I've written about this before (in fact, just a page or two ago). Even those sufferers of eating disorders that have been "recovered" for ten or more years say they fight it off every day. Every recovered individual I've met echoes this notion.
It's nothing more than a set of mental conditioning. You conditioned your mind to believe in a certain ethos/mindset, and now your only real option is to defy it entirely. Imagine telling a devout Christian that there's undeniable proof that his religion he'd practiced for every day of his life is wrong, that there was actually another deity standing right in front of him that created the universe, met him, and shook his hand. Even if he came to accept it, he would still have all those days lived in the Christian faith with the beliefs, notions and experiences he'd had in this mindset. Through a combination of conflicting ethics, nostalgia and defeatism, he might constantly question if perhaps his old ways were correct after all, even in the face of obvious proof.
Eating disorders are a crutch as all mental disorders are; you become addicted to notions and behaviours as you would any substance, not through chemical messages, but through the power of suggestion. They say that a mental disorder is your "inner mother", rather scarily, because you replace your nurturing mother with a mental fabrication of a caregiver. As we grow up, most animals shed their guardian and start anew, but humans are more complicated, emotional creatures and we crave that level of intimacy. We create the inner mother to pat us on the back whenever we do something we deem is "right", and scold us when we do something "wrong". Only this voice is entirely our own lie, and we're laying out our own gridlines for what we believe are "rights" and "wrongs". You grow addicted to the inner mother's praise, and start to control situations to hear the praise more often. You start to change the criteria and the rules of the world to your own will, and suddenly there's a whole bunch of "rights" and "wrongs" that make absolutely no sense to the rest of the world.
So long story short, we've created a new set of rules for ourselves that make little to no sense for our wellbeing, but grow addicted to obeying the rules to feel the worth of praise.
Jazzy, as for your own dilemma, two things. Actually, THREE things; the first is - did you know that 60% of male personal trainers that premeditated this as their chosen career did so because they were unhappy with their bodies, personally? That's more than half of all personal trainers with some form of negative personal body stigma as their drive. There's a high chance he personally feels more conscious of himself than he lets on and that he empathises with your situation pretty damn well.
Secondly, you noted two things and repeated those two things - you "feel fat, and worthless". Do you realise what you've just done? You've underpinned one of your most obvious clues to healing yourself, and that's the fact you've bunched these terms together almost exclusively. Somehow, you're making a particular link between the two as a cause and a result. Do you feel fat BECAUSE you feel worthless, or do you feel worthless BECAUSE you feel fat? It must be one of the two, otherwise you wouldn't have subconsciously typed it in such a manner.
If it's the former, you feel fat because you feel worthless - you're signifying that you feel your body size, shape and form MUST be undesirable because your mental state is askew. It's a common sensation - when you're in a foul mood, for example, you automatically assume you must look worse. Even someone without a mental disorder will have angry or sad days where they'd rather people don't see them - not solely because of their emotion, but because they believe their sadness or anger might cause their form to change negatively. Now, we know this isn't true. Save for a few mascara tear-marks or frown-wrinkles, you look exactly the same. But a mentally-drained state can make you believe you look different.
If it's the latter, you feel worthless because you feel fat - you're very simply putting "skinny on a pedestal", and determining worth by how slim you are. I wouldn't hesitate to suggest (but would never imply) that you perhaps found it endearing that your new man had this insatiable drive for fitness because it was a guilty notion of yours that you could excuse having ED-related thoughts again in order to attain a similar ideal. ED-sufferers, believe it or not, are very liable to find health nuts and drug addicts attractive for the same reasons. An obsessive or self-destructive mindset in a partner becomes a very desirable trait as it lends itself to be an excuse to allow you to indulge your own. But whether or not you have been exacerbated by the notion your partner is a fitness fanatic, if this instance best reflects your situation you're clearly still very much fixated on an "ideal form" for you, which is actually something that does not really exist. You are already the perfect version of you; the current version of you is the one with the most life experience, knowledge and the greatest amount to offer.