I can relate to what you're describing about your first cheeses; in reading more and more first-time posters saying similarly, I think these are common beginning problems.
The sourness/bitterness typically comes, I think, from too much whey remained in the curd when pressed; this whey contains lactose, which continues to ferment down the road, souring the cheese. This could happen from cutting the curd too large, and/or not cooking the curd long enough, and/or not stirring the curd often or hard enough.
Another problem I have had and seem to see is that aged cheeses are too crumbly and brittle. This usually comes from developing too much acid in the vat (vs later, in the aging process), by cooking too long or too low, allowing the starter more time or better conditions to product too much lactic acid. (The acidity helps leach calcium from the protein network, which makes for a crumblier texture; by contrast, cheeses with higher pH/less acidity retain more calcium and have a more elastic curd).