Sweet Pea Leaf Damage

There are two possible causes for this condition. One, and the most likely, is a tiny leaf miner. These are the larvae of equally tiny moths and they tunnel in this erratic pattern through a leaf, leaving scar tissue. In small numbers, they are rarely a problem, but these have clearly gotten out of hand. They are very difficult to control because they are inside the leaf but look for Bt formulations that might help. The second is one of the many versions of bean beetle which also leaves evidence much like this. These can be controlled with sprays specifically designed for beetles, check with a local nursery or the extension service for ones for your area. They can also be pulled off by hand.
And, in consolation, you can consider replanting, it is not too late in your area.