I just dont want to believe it though as he can be so lovely, He's going through stress at work.
Sorry, love, what you're doing is called 'justifying' and 'rationalising'. Making excuses and trying to think up reasons that might make his behaviour make some kind of loose sense.
You've hit the nail on the head when you say you 'don't want to believe it'. There's a part of you that knows it makes sense that he's gaslighting you; a part of you that knows you're not really over-sensitive or unreasonable, but it feels like horrible and mean to think that someone you love or care about or have trusted is actually gaslighting you.
It's hard to accept that actually maybe he doesn't really care much for you at all and is disregarding your feelings and then telling you that you're wrong for just wanting basic consideration and respect. It can feel easier to come up with excuses or reasons why you're wrong or 'just not getting it'.
I would like to warn you that you can do this for a long time and you will get sadder and more confused and more resentful, possibly to the point of becoming unwell. Eventually you will get angry enough to leave and he will act like an absolute victim, or he will dump you brutally one day and tell you it's your fault because you're impossible to live with.
You can't make sense of where you're going wrong, because you're not.
You've started the right way. If you feel confused, write down the confusing situation, line by line. Question yourself. Did you forget anything? If you remember another detail, add that. When you feel sure that you've been as honest as you possibly can with yourself, look at what you've written.
Don't second-guess yourself. Decide what YOU think seems right and fair. Focus on your own feelings. TRUST YOURSELF. Validate your own feelings - tell yourself, 'Yes, what I am feeling is fair and reasonable' - don't give that power to him. You are responsible for your own feelings, not him. You answer to yourself, not to him.
If you think he's gaslighting you, if you think that the way you're seeing it makes sense and you can't understand where he's coming from (unless you try on the perspective that he doesn't really care about you and is only thinking of himself... and then what he's saying does make sense), then you are right to refuse to accept the behaviour as okay.
You have to start to value your OWN judgements about yourself more than his judgements about you.
Surely you don't like being with someone who treats what you're saying as irrelevant or wrong, berates you into going for a walk that you don't really want to go on and have clearly said so, and who then refuses to believe you when you say you're feeling unwell and tells you that you didn't make it clear that you didn't want to go?
(As an aside: surely a grown man can go for a walk by himself if he wants one, and doesn't need his sick partner to come with?)
If he can't listen to you when you speak, I'm sorry to say it but he really doesn't care about you. That's not a poor reflection on you, as much as it might feel like it. It's a reflection of his incapacity to care. It really wouldn't be any different if you were smarter or tougher or prettier or whatever. Some of the most beautiful and smart and tough women in the world have been treated badly by their partners. How you treat people is on you. How he treats people is on him, and he isn't treating you well.