Your envy of your SIL doesn't sound personal, it sounds like she just represents everything you wish you could be. You definitely need to keep seeing a therapist to help you build on your confidence - if your kids are still small, they will be sapping your energy and making you question everything, because nothing is ever finished or perfect when you're dealing with small children!
Also if your SIL's life looks perfect, then I would bet good money that she's struggling with her own self-esteem issues and putting a lot of work into maintaining a "perfect" image. It's hard work holding it together all the time, and a lot of things need to be swept under the carpet and not dealt with properly. You are confronting your problems and handling them - it might not make you look perfect, but it makes you sound like a survivor who can face problems head-on.
I've known a lot of "perfect" women, many of them lovely and sincere, but all of them experienced some kind of implosion in their lives, because if you focus on keeping the surface looking pretty, you don't have time to handle the real gritty problems in the foundation.
Maybe keep some distance for your SIL for a while if it's draining you to hear how amazing her life is. If you feel like you're in competition with her, she might be feeding that to some extent, to make herself feel better. When you do need to see her, take some time to prepare so you look good and feel good. Yes, it's superficial advice, but it doesn't hurt to boost your confidence before you meet her.