I understand what you are saying. I have had similar responses people about how its all in my head, how I just need to get on with it, etc etc. I still get those responses from people I don't know well, but not from people who are close to me.
People I don't know well, I don't tell about being ill, and just make my excuses - or I tell them I have a chronic illness, but not what it is. If they ask, I have learned to say "I don't like to talk about it". I used to feel that I had to tell them if they asked, but why should I? Its very personal, and its your information to offer, not theirs to demand just so that they can judge whether you are adequately ill in their opinion.
I still do as you do - pretend I am not unwell - in certain situations. For example if I am around people who do not know me well. I find I can push through if necessary, but always suffer afterwards, so I try not to do it unless really necessary. Life is so much better now, because my health is more stable and I make my own decisions and judgements about what I can or cannot manage, and I base these decisions mainly on what is right for me.
In the privacy of my own home, I am able to be honest about how well I am. My DP does not question how real it is and whether I really need to rest. There are men out there, who will give you the respect you deserve and accept what you say as the truth they won't expect you to change something so fundamental and unchangeable as how well you are, for their benefit. It sounds like he is able to be understanding, as long as its not inconvenient for him. Please don't believe that all men are like this.
I am not telling you that you should leave him or ditch your friends. But I want you to know that it doesn't have to be like this. Nothing will change however, unless you have higher expectations of the people around you. That will probably involve some losses - of people who cannot meet those expectations. As painful as that would be, I am sure that you will be happier and stronger in the end. Wouldn't it be brilliant if you could make plans around how you think it would suit you/affect your health, rather than what other people would think/whether your DP is happy etc etc
BTW you don't have to justify and explain your illness to everyone all the time, if you cancel or say you cannot do things. You just have to say "I have a chronic illness and I cannot manage that at present, but thank you for the invite". People don't need to know exactly why, what your symptoms are, what your illness is. You can add further information in as you choose, and answer questions if you feel you want to. But its not necessary. If people judge you and think they know better about how your illness impacts you, then don't bother with them anymore. Its so hard, I know. And it can be very lonely at times being ill, because so many people just don't get it. And having an illness that people think is not as real as other more worthy (in their opinion) conditions, makes it so much harder. But there are many of us out here.
I find, even with people who are okay with me, I rarely actually talk about my illness. People are really funny about it. I am not sure why, but it makes people very uncomfortable and awkward. I have realised over time that it doesn't necessarily mean they don't believe me or don't care though. But if anyone tries to pressure me and say the things your friends have said to you, then that's a different matter.
Sorry for the long ramble!