Maybe u do it d wrong way..:/
Try doing them on your knees first. If that's still difficult, try doing incline pushups or even wall pushups. Those pushup exercises are where your upperbody is higher than your lowerbody. Try doing them on stairs if possible or a hill. Once you develop the upperbody strength then increase the difficulty. Try also strengthening your tricep muscles, shoulders, and core. Hope this helps your issue.
As your stabilizer muscles get stronger, you will not that your form will improve. Strengthening of the chest, bi's, tri's, and lats will increase the amount of push ups you will be able to complete. Modify them, for now, but don't give up on the push up or the pull up. Hope that helps