I'm ruining my knits with rubbish sewing together.. please help

[Houseplanter]

So I consider myself a fairly accomplished knitter but my sewing together is shocking.

I was taught to oversew seams but have never been happy with it. YouTube suggested mattress stitch which I prefer from the right side, but the seams are very bulky for baby clothes, which I'm currently making

Does anyone have any advice please?

This won’t help your immediate problem but try to knit in the round for minimal seaming.

Have you tried mattress stitch but only half a stitch in from each edge rather than a whole stitch?

Backstitch everything.

Old-fashioned now but much easier than mattress stitch and gives a v secure seam.

I use mattress stitch for some things over stitch for others.

try starting and ending the purl row a knit stitch, this makes a small bobble for want of a better word. When overstitching pick these bobbles up with the needle it makes a lovely flat seam

Thank you.. I'll give these ideas a try

An easy method is to crochet seams together, using one loop from each edge to create a nearly flat seam;