Recall that mRNA vaccines work by forcing your cells to produce spike protein, which is then detected as an infection leading to those cells being damaged or destroyed by the immune system. If you have already fought off COVID then being injected with the vaccine can only damage your body. It cannot help because you are already immune. And it's not the case that your pre-existing immunity means less damage: it means more, because when your body has never seen the spike protein before it has to build its own immune response in parallel to the mRNA particles degrading and the spike protein manufacturing falling away. The effect of the vaccine doesn't last long so by the time the immune system is fully ramped up the spikes aren't being made much anymore anyway. But if you are fully immune already, then your immune system can start destroying the mRNA 'infected' cells immediately, so far more will end up being attacked.
The more you investigate exactly how these mRNA vaccines work, the more questions are raised. And unfortunately there are remarkably few answers out there in the literature.