Hi
@Toby Griffiths and welcome to Dementia Support Forum our friendly and helpful community. I am sorry to hear about your situation. No-one can say for certain whether they are at greater risk than the person next to them of developing dementia. I am no expert - and it differs depending upon the type of dementia - but my understanding is that hereditary factors play only a small part in dementia risk.
There are plenty of other risks one could worry about in terms of health conditions everyday activities and external factors over which one has no control. If you have a feeling that worrying about dementia is irrational, then maybe it is.
Now that you have joined our community you'll find a mixture of good published information alongside a wealth of direct experience that our members have accumulated. I am sure that you will find this Forum helpful and supportive.