There's quite a lot to unpick here.
1) Rates of regret for transition are lower than most other medical treatment. You're applying a lot of scrutiny to regret, which is fine, but you're only applying that scrutiny to trans care. You're not applying the same scrutiny to eg cancer treatment.
2) Why is regret such a horrific prospect to you? You see someone going through gender affirming care, and then regretting it, and then being forced to live with the result, and this is horrific to you. I don't understand why you don't apply the same standard to the people who are denied access to gender affirming care - the people who are forced to live with the results of not getting the care they need. Why is one situation so terrible and the other is good?
3) In general we allow capacitous people to make choices about healthcare. We tell them what's involved, we tell them about the risks, and then we let them make the decision. We allow people to make unwise decisions, even if those decisions will cause their death. If a person has a necrotic gangrenous leg their doctors will say "we need to amputate this or you will die" but that person is allowed to say "I'd rather die than have you amputate my leg". Why are people okay with bodily autonomy unless it's a trans person getting trans healthcare?
There's also quite a few points you've made that are based on out of date, or misinterpreted, information. Stuff like bone density has been debunked - trans people have reduced bone density before they start gender affirming medical treatment. At the moment people think that the bone density problems are caused by lack of access to sport and exercise.