To me it sounds like there is more going on and you are anxious perhaps about other things and over-focusing on a non-life threatening thing from the past. There might even be a touch of OCD or intrusive and obsessive thoughts about this issue?
If it is anything to do with the latter, getting lots of reassurance about the dress and your appearance might not actually be the way to go. It might be that you have to cut out reassurance seeking. This is very hard I know and it's only for this reason I'm not going to focus on the dress itself.
You know that this is not a huge issue in the scheme of things and you sound like you really want the loop of obsessive thoughts to stop on this so perhaps step one is not to feed the obsession by asking for reassurance and opinions as the latter can feed the obsession. Then maybe look into some online CBT and some techniques to try and stop ruminations. There are various techniques talked of on here that might be useful. Set yourself a "worry time" for half an hour later in the day. You can then say every time it starts "I'll park thinking about this until 6.30" or whatever time you decide. Then sit down then and do your worrying, write it down even. And after a set time, stop and go and do something else.
The weird thing with this technique is that often when you get to your set time you don't feel the same or worry as much. Then slowly reduce that set window over time.
Other techniques are to try and stay in the moment. Feel the ground under your feet, the chair you are sitting in, take in the details, sight, sound, touch of where you are in that moment. This helps get you back in the present and out of ruminating about the past.
The other thing that can really help is just to say to yourself "i did my best. I can't change the past. I can only learn for the future". This might not be that relevant to worrying about a dress. But basically we can't change the past, and ruminating on it can be so very destructive. We did our best is an important thing to remember because we aren't perfect (we'd be inhuman if so!) and we can't always control the outcome of everything even if we try our best. It allows us to accept that we can't be perfect. We don't have perfect information. We do our best.
This doesn't mean your dress went wrong by the way! It's just a technique to get our heads out of going over and over and over something we cannot change and the self-destruction that can have on our present and future lives.
You talk of a body image problem. I don't know about those. But I do recognise something of the obsessive focus and anxiety in your post. I would love for you to be able to release yourself from that. It's so debilitating and anxiety can grow if it gets a hold. Some have suggested councelling. You may or may not be able to access that. But I'd for a start look into CBT for anxiety. But there are huge numbers of people who claim to do CBT who do all sorts of things that aren't really. So if I did seek out an actual therapist I'd go to the BABCP register.
Good luck OP. Remember, the pressure that tells women they must be like "perfect" semi celebrities on special occasions, is one that can suck confidence from people and takes money for that lack of confidence. There's so many great things in life, so many interests to pursue. Every time your mind returns to the dress, tell yourself you're great as you are, and go and pursue something that interests your mind or gives you joy now instead.