I've felt how you feel and no amount of intellectualising it, rationalising it, talking it over to try and make sense of it helped at the time. And I struggled with that because I 'knew' it was a pointless/ridiculous/unhealthy way to feel but crucially felt unable to shake it off using all my usual techniques. I am generally not a jealous person, it felt very alien.
And I think that's key, here. Jealousy is irrational and while you need to take steps to look at why you feel the way you do and try to change your perspective, you should also be a bit kinder to yourself in working out what this is about and why this person pushes certain buttons for you. 'Don't be silly' is not a helpful response.
My situation involved a close relative of DP, someone who I had never felt particularly warm towards - or perhaps had always felt they weren't very warm towards me. Her pregnancy was as a result of 'not trying, not preventing' and 'happened much quicker than they thought'. Announced at 6 weeks to close family, as they're all boozers and thought it would be too hard to keep quiet. At this point, we had been trying for 15 months I had had an early mc six months before and wasn't yet pregnant again and I was GUTTED. I knew that her being pregnant or not had absolutely no bearing on my situation whatsoever - either way, I was still not pregnant, regardless. I KNEW that. But it was awful and I kept my feelings bottled up for weeks, trying to make sense of them before I blurted it all out to DP (and shouted at him for not realising how I might have felt!). For about the first time in my whole life I wanted to shout out 'why me/why not me?' - and yeah, I totally knew what an idiot I would sound, but the nano-second instinct was still there (I didn't 
). For the record, I am no spring chicken and plenty of my friends had got up the duff and now have multiple kids and not once did I feel this way.
And then I got pregnant too - and some of those feelings were replaced by (yet more irrational ones) the thought that his family would think we were 'copying' them (I am embarrassed by my delusions, writing this...). The fact that I would probably mc and things for them would be fine. She'd have a perfect, intervention-free birth with bluebirds circling and baby rabbits hopping round the delivery suite and the child would be walking at 2 weeks and I'll have a third-degree tear, all the drugs under the sun and drop it on its head at three days old. I could go on and on. I should point out that although these things have crossed my mind, I've been far, far from wringing my hands and thinking about it 24/7.
Fast-forward, I am now 30 weeks and their baby is here and despite the fact I had already decided to make a concerted effort with contact and sending presents etc, it's actually been no effort at all and a good bit of bonding. At some point, events in your own situation will naturally eclipse all of this. Your perception of their situation is very probably not the same as the reality - and either way, does that matter? Of course not. This whole thing for me - and for you, I suspect - is nothing to do with the baby and quite likely nothing to do with any of the cast of characters we're projecting it all on. It's deep-rooted self-esteem stuff and the best way of dealing with is definitely to concentrate on you, being the best version of yourself that you can be, taking responsibility for yourself and no-one else and the other stuff will fix itself. Control the things you can control and think ahead a few months and imagine looking back at this time - then try really hard to shape the now into what you want it to be.
And massive congrats 