Deep breaths. Almost everyone panics when they see that second line, even if they’ve been trying for ages (trust me - we spent a looooong time ttc after our first, and I still had a moment of pure panic when I finally got a positive!)
Babies really don’t need much space. Although you can move them into their own room at 6 months (which is still more than a year into your future!), they don’t have to. We chose to keep our DD in with us till 18 months as it was easier for us.
How do you feel about getting married with a 4 month old? You can absolutely do it, but it might take the focus off you, it’s a tricky age in terms of feeding/sleep. Would you consider a shift of date to either earlier or later? A younger baby sleeps a lot, an older one can be more easily entertained by a relative. Don’t know if you have any flexibility with dates. If you want to stick with it, obviously do so, you’ll work round it, but it might change how the day plays out.
You won’t be a failure of a parent. You’re 25, you’ve got a bit of life experience under your belt, a stable partner and a stable job with some savings. While it might be a bit earlier than you’d planned, you’re in a good position to give you child plenty of security,
Best of luck! Being a parent can be really wonderful! (But sleep deprivation is shit. I won’t lie to you on that score. It’s totally shit, but it doesn’t last forever)