I wasn't quite as close as you, but my first baby was just over 8 months when I got pregnant with my second child. I'm our family's income earner, and my H stays home with our kids. I was pretty freaked out, and still nursing, and bed sharing, etc. etc. I feel like it's easy to fall into the trap of worrying what your previous child will think about a new baby (in my experience, he will very quickly have no memory of life without a younger sibling) and worrying that the pregnancy will ruin your breastfeeding - I was told repeatedly that you can't keep bf'ing when you are pregnant, and well, clearly that's not the case. My dd kept nursing throughout. Having said that, when I was about 10 days away from giving birth (at 41 weeks) my dd suddenly pulled off the breast one night, and gave me a look and pointed to her crib and that was that - she never once asked or seemed interested in nursing again, and even when a new baby showed up and was nursing days just later, she seemed curious about the baby, but totally disinterested in the boob. (which was fine by me - lots of people have positive tandem nursing experiences, but I would have been overwhelmed). So I think there's no need to try to prepare your second child, and if you keep your nutrition up, your milk supply shouldn't be too impacted by another pregnancy.
Things that weren't so great were that I definitely don't think my body was back to pre-pregnancy (or even first pregnancy) shape. I was really emotional and hormonal in my second pregnancy, and I was exhausted from trying to work full time and parent full time every weekend (I think I was a bit in denial). There was a lot more crying than I usually do. I overdid it with walking and things like lifting and carrying firewood, and carrying around my kid, and I paid for it, big time, with super bad hemhorroids through much of my third trimester and a generally much less happy perineal area (including brutal roids) after the birth. I freaked out a bit on the first day I was home from the hospital, and my partner had gone to work (we needed some income) and I realized I was on my own with a 17 mo and a newborn and my body felt just awful. I had a panic attack and started crying hysterically and couldn't get out of bed. thankfully my mom showed up, and my husband came home for the next couple of days. I think some of it was coming home too early, some of it was the post-partum hormonal blues, and some of it was just not recognizing what the hell I was up against, until I was actually up against it.
BUT - it got better, waaaaay better. Granted, I didn't also have a 2.5 y.o., but just think, that 2.5. yo will be 3, and 3 year olds are actually helpful! They like to get bum cream, and diapers and wipes for you. they entertain their smaller sibling. The can play playdough for at least 15 minutes. I found that my coping strategy was to seriously reduce what I expected to do each day, to find a play group that I could take my daughter to, and I downloaded Finding Nemo and my 17m.o. had enough of an attention span that I could get the baby latched and happy until dd got bored with the movie, and then we could read books while I nursed. It's not easy for the first few months, but when those kids are a tiny bit older, it will be awesome to see how close they are.
Good luck, mama! Keep the faith, and remember to put your foot down about doing anything that will potentially damage your body during pregnancy, because you need that body for birth, and for after. I read something about a 5 days in bed, 5 days on the bed and 5 days around the bed plan for postpartum, and that was one of the smartest ideas I had ever heard - especially for a third child. and as for your partner, if this is your last child, at least it is getting it over with more quickly