Don't worry about feeling bad when receiving criticism, it's natural! When I went to art school I got absolutely grilled by my lecturer, it sucked at the time but I think it's helped in its own way. Even now I still get a little bummed if something I draw is criticised, but I just need to remember that it's usually for good reason. So don't worry, it's all part of progression.
SAI has a very handy tool that can help your lineart, so it's a good choice of program! Since you're not using a tablet, I'd recommend using the linework layer option, the button is right next to the new layer one. It's essentially a unique layer that lets you create vector lines that can be adjusted very easily with the edit tool, it's great for drawing with a mouse. Hopefully that helps you, it made a huge difference to my lineart when I began using it, and even now I still work with it. Good luck, and remember to have fun!