I don't think someone only gets 20 min bottom time, at those depths, for not being completely streamlined or having bad fins, in fact unless its faulty reg/tank, I doubt its equipment related at all.
The above suggestions certainly will help cut down on time, but 20 minutes bottom time points to some other root cause. Some of these have already been addressed...
I would like to know how many dives you have so far? Do you find yourself being really anxious and breathing heavy for no apparent reason? Like someone else stated, are you fighting current and ofcourse, what size tank are you using?
Also, your physical make up / fitness will have EVERYTHING to do with this time also.
Please provide us with more detail, but if this is the only reason you want to give up scuba diving, then let us try and help you. There are solutions.
J