I suffered from infertility and I so can understand how you feel. The overwhelming desire to have a child brings an intensity to negative feelings that even most of those experiencing it struggle to understand.
Like most, I took becoming a mum for granted and was lucky to have two children with my ex. We separated, met a wonderful man who had waited to meet the one to have children with. I very much wanted more children and thought I'd hit the jackpot, even more so than despite being 38, I fell pregnant first month trying. It led to a miscarriage and then the fall to the abyss as we discovered that my fertility was seriously compromised as well as OH sperm count and quality being low and that me falling pregnant in the first place had been an absolute miracle.
I found myself where people suffering from infertility end up. Obsessed, depressed, jealous, envious and bitter, of course all well hidden except to those close enough to understand that those feelings don't define you as a person.
My slap in the face didn't come from work but from my ex. He was a crap father (loving dad, but only taking on the fun part of parenting leaving me all the duties and responsibilities attached to it, including financial as he didn't pay a penny) and got together with a mum whose children lived mainly with their dad but she claimed benefits for them as a single mum. They had together four children and didn't support any of them financially but that didn't stop them trying for a 5th one and at the exact time OH and I were told that her chance of becoming parents together was almost non existent and we had to decide whether to invest in ICSI knowing that with my age, our chance of a successful pregnancy, after 3 attempts, at a total cost over £10K was still below 20%. We decided not to go for it.
The sense of injustice and bitterness ate me inside. Experiencing listening to my kids' excitement every time they came back from their visit was crucifying and all the efforts to pretend I was happy for them exhausted me. I dreaded the announcement of the birth, and then seeing the pictures that my kids wanted to share.
What I did find though that going through it actually made it easier o get over the pain because it became normality. The people I thought the less deserving of being parents were so and accepting it and dealing with it however painful made me grieve quicker and indeed, gradually, with time, as I went through my own battle, the negative feelings slowly receded as I started to focus on the positives of my life.
This to the point where I felt no envy, resentment or jealousy at all and could even accept this little person in my life as she became part of that of my children, and a year ago, as she was 4yo, I even welcome her in my life and last Christmas, as I happened to come across a present on sale which I knew she would love from my children, I bought it for her. The sense of appease that came from the realisation that I could do this was immense. It made me grow as a person and made me realise that we are so much stronger than we think we are.
A long post to say OP that you might find that supporting this person might be exactly what will help you with your own battle. It will allow you to see that although she's got what you want more than anything in the world and that it is unfair that she should be the one getting it, her happiness with that gift won't be the same happiness you would experience. She will have her own fears, battles and resentment to go through too and you might find that you are the best person to help her through them because helping her might actually be your way to defuse all your negative feelings that you are keeping inside.
All my heart goes to you and hope that whatever life brings to you, you will be able to find peace and happiness within it.