Weak Toenails

Im new here and I just wanted to ask you all a question. I am very interested in nails, and know a little bit about them, but lately I have been finding my toenails are very weak whenever I remove my polish. I use reasonably cheap polishes, and usually leave them on my toenails for a few weeks at a time. When I remove it, I have found chipping all over the nail, not just the edge, and large bubbles in my nail that I have proceeded to cut off. I have no idea why this is happening! I think it may be that I leave it on for too long. I never have this problem with my fingernails. I recently gave them a break for a couple of months from polish, but as soon as I applied it, it happened again. I use base coat, 2 coats of polish and topcoat. Can anyone help? Thanks in advance?

Corinne