I know exactly how you feel. I seem to "forget" things I learned in the previous semesters. I think a lot of it has to do with study habits and like other people said, desire. If your desire is only to make good grades then that is one thing, true understanding is a different thing.
That being said I don't think they are 100% separate. You've got to have confidence. You know a lot more than you think! Taking one class will never make you an expert in things and the speed most courses run at I don't know if there really is enough time to truly understand something. The reinforcement of your grad level courses will help you recall things and further understand them but I'm convinced it takes real world experience and application to ever truly consider one's self an expert in anything.
Sounds you've got the desire and the background needed, nothing standing in your way except yourself and probably a little luck too.
Best of luck to you in achieving your goals!
(You've inspired me to look over my old notes from Thermo, Heat Transfer, and various other courses that I feel like I've "forgotten" so much of).