Sorry that you find yourself here friend. But you did find a great place for support.
Your wife didn't cheat because of the weight loss. She didn't cheat because of the "new found" beauty. The reason that she cheated, generally, is because she has issues that need to be addressed. It is possible that the same issues that led to her weight and self esteem issues, are the same ones that allowed her to cheat. This will all have to be investigated further.
The bottom line--your wife has decimated you. Due to her brokenness, she has destroyed any and all trust that you had in her. Trust that was built up over a lifetime, has now been shattered. And lets not mention that your self esteem is in the gutter right now. Maybe you feel emasculated. These are all normal parts of the fallout of infidelity.
BUT, if your wife does put in the hard work, and you are willing to attempt reconciliation, you can build a new marriage from the ashes of the old. The most important thing to learn here, for both you and your wife, is that this can not be ignored. You can't sweep these problems under a rug, and hope it goes away.
Infidelity is a process, and must be dealt with thoroughly. It will be painful to endure, but it will get better. You have to understand that the affair is all on your wife. There is nothing that you did or didn't do that caused this. And it is up to your wife to discover, and solve, these issues. Yes, it will hurt like hell, but what other choices are there?
If infidelity is a dealbreaker for you, then that is just the way it is. No one will blame you for divorcing, if that is what you feel that you need to do. Your wife already left the marriage---it is up to you if you want to attempt to help save it.
Keep reading and posting. You will get a lot of advice. The members here can really help.
2 boys-18 & 21(special needs)
Married 23yrs.(together 28yrs.)
All work and no play has just cost me my wife--Gary Puckett
Acceptance the Marriage is over: 7/2/14
Heading for D