So...There is a man that always assumed he, himself, was a woman. That's a trans person, right?!
Now, we have two possibilities. Them undergoing the sex change surgery and the accompanying therapy and whatever else is part of it (it depends on country and doesn't really matter much further in this post anyway) or them not undergoing it and accepting the fact that they have the opposite sex.
1) If they underwent surgery then the matter is actually clear. They WERE bothered by it. They life WAS negatively impacted. The surgery was their "cure" in much the same way as it is for physical ailments. The only difference here is that a physical change is for the sake of aiding the mental sphere of someones life. But you know what?!...That's not even unique. Women that undergo mastectomy sometimes fall prey to severe depression, loss of self-esteem etc. Their cure is a surgery that gives them back the breast or breasts that were previously cut. Well, a fake one, not the exact same. And their depression often completely disappears after that. And I'll have you know, depression is without a doubt considered a mental disorder in every part of the world by pretty much every doctor. The most common mental disorder in the world, in fact. So yes, surgery whose purpose is to make a person "feel good" is a proof of them having a mental issue before it.
2) If they live in a male body they have to deal with the consequences of that. They will have to use the toilet for men whenever they go to school, shopping centers etc. They will be spoken to by "mister", "sir" by everyone they didn't meet before (and apparently that is an issue, it is this threads very point). They will have to write they are male on any document that requires that information. They will not be allowed to take work that is reserved for women only (employers often prefer women for certain roles, like hostesses, but it's a must-have for actors that play women too, obviously). Even if the law somehow allows a person to be considered a woman despite being in a mans body thus allowing them to enter the womens toilets or signing documents as a woman, they still will not be allowed to undertake those jobs (since those usually are based on body, not documents) and they have to actually go through the process of that being accepted. It's an equivalent of a surgery.
So...No matter which way I look at it, a person that is trans have their life quality being negatively impacted at some point, whether or not they accept themselves. Even if they accept themselves and shrug at the consequences of that, they are likely to have moments when they will be more susceptible to that popping up. And if they don't?!...Um...Are they actually trans people then? I mean, if a person feels perfectly fine living in a mans body a mans life...can they really be trans, can they really consider themselves women, in the first place?!
In my opinion it seems like you are denying the trans people the consideration. You are blindly pushing this philosophy of yours without any real care into what it means for the people you'd like to think you "defend". This is part of the reason why feminists or other activists are so looked down upon. They often cause more harm than good to those they are "standing up for", all the while getting on the nerves of everyone else in the process.