Am feeling very similar. DS is my first and is 3 months old as of yesterday. I am definately mourning the loss of his infant look. He's growing up so fast and that makes me happy in one regard, but sad in another.
I had really wanted an unmedicated, vaginal birth with him but it didn't turn out the way I'd hoped. I was able to labor for 18 hours unmedicated but then, despite doing all the things our Bradley coach had told us to restart labor, my labor stalled and I was forced to do the pit/epi route. After several more hours of that, I was at 8 centimeters and wouldn't go any further, at which point my OB ordered an emergency c-section. I was SO disappointed as I'd really wanted the experiene of a vaginal birth.
I wouldn't mind having at least one more child within a couple years or so, but more and more dh seems to be content with just the one child. The less he seems interested in having another baby, the more saddened I become that I may never get to experience the vaginal birth I so desire.
In addition to all that, I'm also disappointed in my body right now. I had really wanted to EBF but DS didn't urinate w/in the first 24 hours in the hospital and we were told that we had to give him formula (REALLY wish I had said NO). He'd had latch issues to begin with but once he figured out that he could get food w/out working for it, bfing became a major NO-GO for him. It got so that all I had to do was take the breast out and he'd start screaming. I tried pumping for awhile, thinking that at least he'd still be getting the breast milk, but I just wasn't able to find the time to do it as often as was required. As a result, my milk supply slowly dwindled. I had been hoping to relactate, I even got an SNS and some fenugreek as per my lac consultant, but that didn't work either. I'm just so saddened that DS is exclusively formula fed. It is so what I DIDN'T want.
Now he's having bad constipation and gas problems and I can't help but feel guilty about it. I keep thinking if only we hadn't allowed the nursing staff to give DS that bottle in the hospital, if only I'd made more of an attempt to BF, if only I'd tried harder to make the time to pump, if only my dang breasts would DO what they're supposed to do and make milk...so many "if only's". I suppose I shouldn't beat myself up about it but I can't help it sometimes. Especially when DS is grunting and crying because he's unable to pass stools or because of his bad gas. Poor little guy!

As for your situation, I think it's only normal to grieve the loss of having children once you've decided you're done and aren't going to have anymore. I would give it a year or two and rethink it then. Perhaps your situation will have changed and one more child won't be out of the question at that time. You just never know!
Take care and big hugs to you!
K