How To Overcome These Feelings?

Hi! This post is like a "get off my chest" while asking seeking advice.

My SIL(26) has a one-year-old (F1) and just had her son literally yesterday and I don't know how to not feel sad with a hint of jealousy. For context, my husband (32) and I (31) have one child (F6). We have been TTC for almost 3 years now and like many of us, it's been difficult.

The same month she announced her second pregnancy was the same month I should have gotten a positive pregnancy test (based on my symptoms at the time) and sadly, that was not the case.

I'm just trying not to feel what I'm feeling now but it's so hard. How can I cope with these feelings and not make it obvious that I'm hurting in silence? I know I should go to therapy, and I plan to now that my insurance has activated. It's hard--I feel like time is flying by and I'm aging and the age gap of my daughter's sibling keeps growing and there's very little I can do to stop it. Anyways, has anyone been through something like this before? Please give me some kind words of wisdom.