/u/MrTankt34 nailed the biggest issue, which is the perspective errors.
Don't be afraid to use construction lines when doing technical drawing, they can significantly reduce or eliminate technical errors.
If you're really serious about improving your art, look into picking up copies of Successful Drawing by Andrew Loomis, and How to Draw by Scott Robertson. They both cover drawing in perspective from a detailed, technical standpoint. If you want a single book that will teach a wide array of skills, though not in any great depth, Drawing on the Right Side of the Brain by Betty Edwards covers the core skills of drawing quite well.
The Loomis books are widely renowned as some of the best art instruction books in the world, and are standard at most art schools. The Robertson books are quite new (How to Draw came out in 2013) but have gained quite a bit of acclaim in that time for being excellent technical instruction books. The Edwards books are absolutely fantastic introductory material for people who want to draw, but don't know where to start.
Fundamentals of Perspective 1-3 on the gnomon workshop are fantastic. They are $50 each or $60 a month for access to their entire video library (completely worth it if you are serious about learning concept art/entertainment design).
Alternatively there is a good book that features perspective by Andrew Loomis, Successful Drawing, which itself refers to another book Perspective Made Easy by Ernest R. Norling.
Personally I found most books on perspective to be hard to grasp at first, but after watching someone demonstrate in a video it really clicked.