barking I completely understand where you are coming from. All the time, effort, cost that we invest, and all the heartache and disappointment we get back. There is a breaking point.
I also think that I have reached that point where I really need to ask myself for how much longer I can continue this. To be more precise - I had a major meltdown on Sunday (poor DH didn't know what hit him).
Turns out I have completely utterly misjudged my cycle this time - expected to ovulate on CD10 or 11, as I have done recently, so started dtd daily last Thursday, trying to be super-efficient. Well, I got it all wrong. Positive OPK only appeared on Sunday (CD14) together with all the tell-tale signs of -can you guess- another uti. So not only am I out for this cycle, it's also entirely my own fault 
Cue complete melt down from me
It just really hit me... over 2.5 years of ttc, and nothing to show for it but 4 mcs, all with the ripe old age of 43. I nearly threw the towel there and then.
After a bit of soul-searching, I have come to the decision that I will keep trying until August this year - this will make it 3 years of ttc, and if I am not pg by then, I will sit down DH to have a big talk about if we are going to continue or call it a day.
I feel I need some fixed point to work towards, this is really affecting me and everyone around me, and not it a good way. I can never really unreservedly enjoy myself anymore, I always come back to thinking about ttc, the mcs and all the what ifs.
Right now, I want nothing more than be happy, forget about it all and lead a 'normal' life, without the whole ttc thing constantly hanging over me. I know I can't do that straight away, but I think I can start working towards this, trying to start accepting that I will never have that elusive 2nd child that I so wanted.
I think I will also decrease the supplements (apart from folic acid) - I have been wondering if they have contributed to my ovulation coming forward... Been taking them less religiously in the last few weeks, and my ovulation has moved back to CD14 again. No idea if they are connected, who knows! Yes, I want to improve the quality of my eggs, but right now, I am struggling to even fall pg, so no idea what the hell is going on there. Plus, they cost a fortune.
Seeing the consultant on Thursday about my MRI, which might give us more clues - although I suspect that it won't.
Sorry for the rant - but I've read some of the comments, and they really resonated with me.