How are you at sketching a long, straight line?
It took me hundreds and hundreds of blank pages filled with straight lines before I could really nail even just a basic sketch of cube. If you can draw nice straight lines you can throw down a little perspective grid for yourself and that will help alot with your perspective issues.
In the first two drawing classes I had at Uni we spent the first two days just sketching lines and lines and lines. I still do pages of lines to warm up. I think it’s pretty important. When you get really good you can just doodle out a sketch but I think for now it’s really important to always throw down a little perspective grid for yourself.