I had a very similar feeling last year OP - I could have written your post. I had just bought our first house with my lovely husband, working in a great job I had spent a long time training for, lovely friends and family, comfortably off for the first time ever. Plans for a baby in the next couple of years.
I should have been cock a hoop, but I wasn’t. I felt totally detached from my own life, as though I had lost the knack of happiness. I felt like the most ungrateful person in the world, because I had everything I had wanted and I felt totally empty, as though life was happening somewhere else and I was missing it.
I think part of the problem, for me, was that I had spent a long time working towards goals (job, house etc) and had achieved those. I felt that should have made me happy, but I realise now that something that makes me happy is having a purpose / goal I am working towards. Once I identified new career goals and put a plan in motion for them, it helped. Same with personal goals - for me that was running, but it could be anything you want to achieve (new skill, hobby, qualification, etc).
I also identified that I felt I was at a plateau - I had gone through the excitement of getting married, buying our house and getting my dream job, but was still a couple of years off the next stage of kids. It left me with an emptiness. I had to learn my way out of that by realising that joy isn’t always looking forward to the next life event. Joy is living in the moment you are in, and appreciating what it brings you. I had to re-learn what I love to do in this moment (spending time with family, reading, gardening, running, hosting parties) and appreciate the things this stage of my life gives me time to do.
I feel so much better now than I did a year ago - it’s almost hard to remember just how isolated and out of touch I felt. I have spent a year practicing mindfulness and deliberately seeking joy - writing out physical lists of things I am happy about, retraining myself to feel positive. It has helped enormously.
I also want to say - don’t be afraid of seeking help. On this occasion I didn’t take anti-depressants, but I have several times in the past. Sometimes it’s what you need to get you back on track, and there is no shame in it. Maybe you won’t need them, but if you think they would help, please do speak to your GP.
I wish you all the best OP - there is no reason why you won’t be happy again, I promise.