Decided to stop collecting stamps today

I grew up collecting stamps because my parents said it would be fun. It was fun for about a week, but then I got bored with it. I haven't collected any stamps for over a decade now, but I've never officially come out as an aphilatelist. That changes today.

Some questions for more experienced aphilatelists:

• Is it okay for aphilatelists to buy stamps for mailing letters? I try not to buy more than I need, but I send so few letters these days that I usually end up with at least half a sheet lying around even when I buy in the smallest quantity the post office sells.
• Sometimes I forget I have extra stamps and accidentally buy an extra sheet. Does that count as collecting? Should I buy a postage printer just to be safe?
• What do you guys do with all your extra stamps after you stop collecting? Mine are mostly all postmarked so they can't be used to mail anything, but I feel bad just throwing them away.