FSB, I got pg when DS1 was a year old exactly - I have 21m between my two boys (DS1 was two at the beginning of April, DS2 is 14wo now). DS1 was also conceived after treatment (not IVF, but Clomid - was told I'd never have children when I was 22, eventually had DS1 when I was 28).
I fell pg naturally with DS2 - it was a bit of a surprise, although we'd been planning to have a small gap if possible - I didn't know I could get pg naturally, so although we "planned" it, it was still a surprise - but a nice one iyswim?
What you might be forgetting in your concern for your DS1 and not spending time with him is that he will soon have a little brother or sister - watching DS1 stroke, pat and kiss DS2 is incredible - he loves him so much and DS2 already gazes at his big brother with a huge grin on his face.
I won't lie to you - I have found it tough, and for a while the bad days outnumbered the good, but in the last couple of weeks, the balance has shifted and it's getting easier. Having a 2yo and a baby is one of the hardest most rewarding happiest things I've ever done and I'd not change it for anything.
I had a really hard pg this time round, with diabetes and working 4 days a week and worrying about DS2 being breech and I had one horrible night when I was about 26 weeks pg where I sobbed and sobbed and sobbed - but after I was diagnosed with GD all went well and his birth was easier than DS1's. I was posting on here 2.5 hours before he was born!
If you want to email me, please feel free - hunkermunker at gmail dot com - I don't know if I'll be any help, but I do have some idea of what you're going through x x x
(Oh, and the loving another baby thing? IME you love the baby as much as your older one - and you see your older one in new and fascinating ways - they're a big brother now and it's fab to watch!)