Revise them!
There could be possibly no other way to overcome the obstacle in your situation.
Sure you may be able to make out on what you already know and express your regret on what you forgot. But what is the end result? You don't remember what you ought to know.
Although this isn't something to be embarrassed about (as this happens to even to the best of us at some point of time), this isn't something you should ignore either.
Going through your ancient notes may bring nostalgia (personally that's why I keep them) but it's intent on effective revision depends on the quality of the notes themselves.
The Internet should be filled with tutorials and crash coursers to bring back what you once knew. It may also allow you to learn more than before too.
Forgetting doesn't necessarily mean memory erasure. Note that if you really did study your subjects with passion back then as mentioned in your post, it might not take long to remember them through revision.
Best wishes to remember your once forgotten subjects and hopefully learn new this in the process too.