You're not being unreasonable at all; until someone has been there themselves they will never truly 'get' just how heartbreaking so called 'secondary infertility' is. It's truly gutting and somehow having conceived, carried and delivered one child just serves to make being unable to a subsequent time even harder, not least because many of the explanations behind fertility aren't so relevant for secondary infertility.
I have 2DDs 4.5 years apart. It's not the gap I would have chosen but I wouldn't have chosen the various miscarriages and the ruptured ectopic either. I reached a point when I began to wish that we'd never had DD1 somehow feeling that if we'd never had her we could have been a childless couple who went off and did all those things we did pre-children instead of being surrounded by pregnant people and ever-expanding families. It was a low, low time in my life and for a while I desperately hoped that I wouldn't wake up so as to avoid the questions about when we'd have a 2nd DC and to avoid the complete shame I felt at wishing away my very precious DD1 
. I stopped being me and I hated every single minute of that time.
After the last miscarriage I was referred by a kind, understanding nurse to an obstetrician in the recurrent miscarriage clinic. She also advised me to change my job. I did that (well, location).
The obstetrician gave me hope and treated me in the most wonderfully human way ever. She arranged for me to have blood tests and various other things that I didn't really need but, because there was anecdotal evidence that pointed to an increased likelihood of conception after the procedure she let me have them done. When I got pregnant for the 5th time in a year she scanned me fortnightly, at my convenience.
DH and I had agreed that whatever the outcome of pregnancy 5 it would be the last and he would have a vasectomy so we could move on as a family of 3. We'd already booked Christmas in Lapland (the ectopic had happened at Christmas) and starting doing things that were only really do-able/affordable with a single child. Life changed when we started celebrating our family unit as it was instead of missing what it wasn't. It wasn't an overnight thing by any means but being able to do it helped. I also became braver about telling people that we couldn't have any more children if they were rude enough to ask. Being definite helped me, I didn't really care how it made them feel because I was the one living my life, not them.
We are now a family of four but I still shed tears over those dark days. DD2's name means 'wished for child' and her middle name, Hope.
I don't envy you the situation you're in; it's a grim one and one that threatens to overtake everything else that life offers. My only advice would be to plan for life as three and do things you mightn't do if there were four of you or if you were pregnant. It's hard, unbelievably hard and it's not unreasonable to wish that it wasn't.