What can a 32 year old mom do who used to be y to get out of a major frumpy slump?

I have been looking in the mirror lately and all I can think is "what happened? I have gained 40 pounds, I feel gross, and I end up wearing my husbands t-shirts and sweatpants. This is so not me. I have recently signed up at the gym and I pray to God that I lose weight soon. Hot weather is approaching and I can't cover up anymore. What is appropriate to wear while in transition that doesn't show too much but looks stylish? My self esteem is shot!