Belief in yourself is paramount. Good physical wellbeing cannot be obtained doing some weights, cardio and food management regimen, there has to be a good mental and emotional balance too.
You need to believe in yourself as a person. You need to recognise your own self worth, and you need to realise that life is going to throw you these things, but it's how you learn to hit the curve balls that counts, and if you stand there and miss, or stand there and just allow life to hit you, with balls you get thrown, it will hurt.
You need to realise you're not second best, his ex is, because he is with you now not her, and if he ever tells you it's not working, and ends it, (I hope it doesn't happen), he will become second best not you, for not making it work properly, and not realising what sort of person I believe you probably are.
He needs to be told how you feel. You need to have trust in a relationship, if you don't tell him, it looks like you don't trust him to deal with it poperly, like he'll forget to say anything to family, or just act like you're paranoid, or need a thicker skin.
So he needs to know how you feel, as telling his family will make him feel, you couldn't tell him, and couldn't trust him to handle it, and that might be seen as a betrayal of trust, and lack of confidence or faith in him, to protect and defend your feelings and wishes.
The flip side of the coin is, you use what's being said to your advantage. Do you really want people comparing you to some skinny sort he's not even with anymore. He's with you for reasons, and if he wanted skinny real bad, he would see any physical flaws in you as a turnoff, which he clearly doesn't. He clearly knows whatever slight human flaws you have, (and we all have them), compared to his Ex, there's more pros than cons, and he's gotten a better choice of person to be with, I.E. you.
You have to want good physical condition for you though, not him or anyone else, and their comments on the flipside could be used to your advantage like I said.
Use the comments as a way of showing them, you can be the sort of person she was physically, but you're going to do it better, and coupled with your human traits, you're going to be head and shoulders above her in every department.
Use the comments as fuel for your fire, make it your goal to look potentially better than she did, and give the critics the shut your mouth they need. Actions speak louder than words, so if you work hard and make the right choices, you could eclipse any kind of look the Ex had, and then the crtitics wil have to shut up, or better still compare you to her as being far superior, not just for looks and condition, but possibly for talents, domestic skills, brains etc etc
What would they do if you looked killer, say oh you probably just had some slimming pills, or starved yourself thin, so what?
I doubt it, and even if they refused to accept your results as hard work, not stupid, unhealthy, or expensive workarounds, that celebrities, or emotionally confused teenagers use to get slim, just say well we'll see if what I did was stupid, when I'm still doing it and feeling great in 30-40 years, and you're all getting up each day, complaining of bad knees, and bad backs etc etc.
For every action there is an equal and opposite reaction, so for every putdown, logically there must be a comeback, in this case either verbally or physically.
Are you going to let them say this, or are you going to take away their reason to compare you?
Work hard, eat right, and give people like us your feedback on your progress, so we can make sure you're getting the right sort of critique for you, and you are being looked after emotionally and mentally by likeminded people.
I would be proud to see you come back here, and tell me how much they looked uncomfortable, or spoke in other ways about you, when they saw you looking killer for the first time.
This is your life, but ultimately your body. No matter what they say or shouldn't say, you need to see the value in you, and the potential in you, make it happen, not just for now, but for the long-term, because whatever you don't do today, in 30-40 years, might come back to bite you when it's too late.
I am sure ou are capable of so much.
Don't be afraid to slip up sometimes, and don't fear yourself or failure. Be strong, chin up, and walk tall okay.
You CAN do this, and you know you can.
GOOD LUCK, and anytime you need more words that's what we're here for, we will look after your emotional and mental sides too

A good body comes from hard work.