No, you never lose your Steem Power. You don't give away power by upvoting another post/comment, but rather you are allocating rewards from the daily reward pool on steemit. Each day there is a fresh reward pool, and its up to the users to delegate who gets what and how much. The more Steem Power you have, the more "say" you have.

Yes, your voting power reduces based on how many times you upvote and by how much. At 100% voting power, you can do 40 fully powered upvotes until you reach 0% voting power. In other words, a full upvote is 2.5% of your voting power. That's why a lot of people use their voting sliders to do half powered votes, or even 10-20% upvotes (thats what I do for most comments, unless I especially liked it). Oh, and every 24 hours your voting power recharges 20%.

Hope this makes sense!