Hi @Duck_Duck_Loose, and welcome to the forums!
Sorry to read you’re having problems. As @GoGirl12 mentioned, worth ruling out anything medicinal or perhaps worth a consultation with GP to rule out any health causes if libido has always been very low.
From what you have wrote around your feelings around libido and also after the occasion, it does sounds likely to be more mind related than physical, and an uncomfortable loop that on the occasions you can enjoy sex, you beat yourself up after. So whether you have sex or not, you’re kicking yourself where it hurts.
There is possibly a lot to explore around this and self-help books may help a little, but I think talking it through with someone will help better to find where it has come from, probably through some form of counselling. It could be something that has come from upbringing, parents views, schooling, any past experiences and our perceptions at the time registers and it becomes part of us.
An example of this for me is with my disability, and of course unlikely to be any similarity to the probs you have mentioned, just the easiest way of me to try and explain how past events mapped out my beliefs. My Dad could never accept his children were disabled, and when diagnosed at the age of 9, our relationship changed completely, he changed as a person.
I never realized until last few years (I’m 43 now), that experience taught me that when others knew I was disabled, they too will push me away and reject me. Being bullied because I walked funny reinforced the message.
It caused me many problems, trying to hide my disability, trying to be superman to cover it up, quite a lot of behavioral stuff around it, the knock on affect was it impacted almost everything.
I learnt to accept my Dad just didn’t know how to handle the situation; there was a lot more stigma and judgement in the 80’s and he was a proud man. It doesn’t excuse it, but he is only human and tried his best.
I only mention this to highlight how counselling helped me understand my beliefs and feelings, and in turn behaviors. Once I knew that I could then work on changing it. Not a quick process, but feel so much better now. If I had relied on self help I simply wouldn’t have understood (let alone accepted) my beliefs, or the ability to try and change it.
NHS offerings tend to be only 6 sessions of CBT, and I think it is unlikely to be helpful for you as from experience it is more a tick box exercise.
There certainly used to be places that would offer heavy discounted sessions with trainee therapists, it may be worth looking around online as with Zoom and such like technology it opens up options.
Trainees will have at least 18 months of training, and like any qualified therapist they will often be a member of BACP, practice ethically, and will be in consultation with Supervisor in the same way.
The other thing may be worth considering is just being intimate with your husband, rather than focusing on sex itself. Candle lit massages with oil, bathing/showering each other, plenty of cuddles. Take things back a notch and see if things change from there - it might take the pressure off yourself around sex specifically and break that cycle of blaming yourself.
I can imagine it is not easy for you or your husband, and this may at least bring you closer together. As @WillC also mentioned, open honest conversations between the two of you might help also if it’s not something that is happening already, and the intimacy may help those conversations become a little easier.
Apols for the long winded ‘pro-counselling’ waffle, wish you all the best, and I’m sure there’ll be lots of other suggestions and ideas from others.