when you say "not as good as it used to be".... when exactly was that? a year ago? 2 years? I can't imagine that your libido/erections got that much worse by age 20.... or are you comparing the way you feel now to the way you felt while you were ON? BIG difference there....don't get those feelings confused....you'll never feel OFF the way you feel ON.
if you think your body's natural test production is "healthy", then the only other explanations would be a) you're not fully recovered from your last cycle; b) your lifestyle...are you eating right? sleeping right? stress? all these could affect your libido, etc.
I still think you should go with something that will help boost your body's natural test-making process and recovery... something that won't suppress you.... Sustain shouldn't suppress estrogen that much, and you'd only run in for a month (1 bottle) and then see where that leaves you.
Couldn't hurt to get re-tested after that....make sure your H P T A is functioning properly, etc.
(oh, and notice that I haven't flamed you yet for cycling at such a young age! *grin* There's a reason why that's stressed.... it's so easy to screw up your natural hormones, that risk shouldn't be taken when your body is at its prime)