Huge sympathies with you here
I have put on slowly but surely 2 stone over the 5 years I have been on thyroxin.
Your Thyroid gland is one of the most important glands in your system, it controls your metabolic rate...there is no real substitute for the thyroxin we produce, synthetic and pig thyroxin will never properly replace the loss, however, not taking thyroxin would lead to hair loss, weight gain, lethargy, issues around the proper function of your heart, and even as extreme as coma.
So NO, don't stop taking it, maybe look at the bigger picture around autoimmune, you are clearly actively autoimmune, is there perhaps another cause for weight gain.
Easy to say but harder to do, exercise is first, supported by diet. When you are actively autoimmune this is really hard I know, I have Lupus, no thyroid and Hughes syndrome...I also love chocolate!! But don't drink, don't smoke.
Try and remember, you are getting married and not just having a wedding, you are loved and cared for, who really gives a toss if you're a couple of dress sizes up!