Help with keeping UV from ripping apart

I have this rough shirt model where I have a seam which splits it into a front half and a back half. What I did is select the front half and unwrapped it by “projecting from view” which worked fine

but now I wanna relax the edges so no parts overlap, but when I try to use the relax-brush, bits end up ripping off.

There are no gaps or anything in the original mesh so idk why when unwrapping, some random bits just end up separate from the main grid. Its all technically just a quad grid.
Is there any way to to make sure the ‘uv-mesh’ stays as one piece? (also wasnt sure if this went in modelling or texturing)

See how on the original around the edges, the gray is lighter? If you look closely, you’ll see this is because there are faces stacked on top of each other. Relaxing the main island will not relax those separate islands, hence your issue. As to WHY this is happening- project from view works great when everything is flat to view, but you have faces that are pointing directly downwards and upwards that are just getting flattened in. Your one seam method works well for the faces pointing forward, but the faces overlapping are along the sides of the shirt. Those faces are not pointing forward and so the UV overlaps.

Try this instead: get the model facing forwards. Select, using B, the faces visible in view. Project those from view. Hide those faces. Get the model to one side. Grab the side faces. Project those from view. Hide those faces. Do the same for the back, the other side, the top, and the bottom. This way, you can avoid the overlapping islands problem.