This is called the "honeymoon period" and it's a well known thing in TRT forums: Most people feel amazing when they start, then that amazing feeling stabilizes out to something much less over months or years.
Disturbing a lot of biological systems works this way: There are a lot of drugs and medicines that make people feel amazing when they first start taking them, but after some time they stabilize out to something much less. This is true of everything from caffeine through benzos, opioids, and amphetamines.
That doesn't mean there isn't a therapeutic benefit, but many people are disappointed to learn that they're not going to spend the rest of their lives feeling as amazing as they did from those first weeks and months.
There are some secondary problems with TRT where long-term usage, especially when the dose isn't quite right (often higher than it should be as people chase those feelings) leads to secondary problems as other hormones are thrown off.
It's surprisingly common for people on TRT to end up with libido issues, for example. Complete opposite of what most people think. Your local broscience master will tell you it's a matter of using a few different additional hormone modulating drugs on top, but most of the time when I see people go this direction they end up with very unstable moods and may never find a solution other than tapering off.
Which leads to the other big problem with TRT: Taking testosterone causes the testicles to atrophy. It's a common joke about body builders, but it's a real thing that happens. A lot of people experiment with TRT for a few months or years and then decide it's not worth it any more, at which point they discover that they can't just quit taking it because they're physically dependent. There are some ways to try to restart the body's systems, but it doesn't work for every situation.