I am in a slightly different position because I don't have children, but I have sought help and have had a c-section agreed before ttc. So it is definitely possible as I've done it and from what the consultant said I'm not alone in having done this. It is unusual but not unheard of.
I won't lie and say it was easy to do. The trouble is if you aren't pregnant you don't fail into a nice neat care pathway. When I went to my GP she didn't have a clue what to do with me and clearly hadn't come across anyone asking for a c-section for mental health related reasons. She wasn't able to help me and put the onus on me to find help myself.
You are in a slightly different situation, if you have had a traumatic birth; there are some hospitals that have clinics for it, so it is more of a recognised problem, though not nearly as widespread as it should be. Which might be something you might want to look into. You may find it easier than I did to get help as a result.
But in my case I fall outside of that option.
In the end I managed to find a fairly local hospital (not my nearest but within a sensible distance) with a progressive attitude to fear of child birth, and wrote to them asking for an appointment.
I didn't get a reply.
So I wrote to the GP and asked for a referral there and to her credit, since she hadn't anything managed to find help for me anywhere else, she did do that. It turned out my initial letter to the hospital never turned up, but they said they would have responded to it, if they had received it. So in theory you can go through your GP or go directly to a hospital.
I now have a c-section agreed, which they have said they will book early for me, if I do get pregnant. Just having this reassurance has made a huge difference to me. I burst into tears in relief when my consultant said "yeah no problem". My fears were causing me such problems that it was preventing me from ttc and affecting my relationship with my husband, but having this agreed has lifted an enormous weight off my shoulders and what was impossible to consider previously, is now very much on the cards.
When my consultant was talking it through with me, he stressed that since my request was based on anxiety, it WAS medically indicated. Please keep this in mind as you go through this. The language that surrounds the subject is frustrating and to my mind is quite deliberate to try and put women who need help from seeking it. Your reason for wanting a CS does not sound like it is 'not medically indicated'. Mental health reasons ARE a medical need. Having this stressed to me by a professional has made a difference to me and how I felt about making the request. It made me feel that my feelings were acceptable and weren't stupid. It had affected my self worth.
However my understanding is that this that sadly its not this easy every where to get access to this level of support. It really depends on where you live. Some Trusts are actively trying to 'reduce the CS rate' and are deliberately ignoring NICE's guidelines for their own reasons. But the guidelines are great in that they are very helpful as they do focus on the aspect mental health and they are putting hospitals under pressure to recognise the problem, but like I say, some places are much less forward thinking than the hospital I have been too.
If you don't manage to find somewhere that is willing to agree to a CS prior to getting pregnant, the other useful piece of information to know that in addition to the better known guidelines, NICE have drawn up some quality standards to back up the guidelines. In these, one of the points is that a woman can request and discuss a c-section AT ANY POINT IN HER PREGNANCY. This is important if you aren't able to get some to agree a CS prior to getting pregnant or find yourself unexpectedly pregnant. The problem was a lot of women were being forced to wait until very late in their pregnancies before they were being allowed to be referred for a maternal request. Naturally this causes a great deal of anxiety to anyone in this position. So NICE are trying to stamp this out and make sure that women can get support and reassurance as early as they need if they are pregnant.
My suggestion would be to do your homework on the attitudes and policies of all the hospitals within a reasonable distance. I chose somewhere which had a 'pro-women's centred care' attitude, rather than somewhere that promoted themselves by saying they were 'proud of their low CS rate' - a term I find pretty obnoxious as it puts a higher value on an arbitrary and meaningless figure than a woman's autonomy and involvement in making decisions about her own care.
I hope you manage to find somewhere and someone who can support your needs and can offer you the reassurance you are seeking.
Good Luck.