Don't worry! Your Steem Power isn't decreasing!
Your voting power (effectiveness) is decreasing by every upvote you cast, but also automatically recharging 20% effectiveness per day.
These (Voting Power and Steem Power) are not the same! These are two different things!
I see that you have 100% voting power now.
Beside ChainBB, you can also check your (and everyone else's) voting power on SteemD or SteemDB and some other places.
You don't have to buy Steem Power to be able to upvote things!
What more, If you upvote something, you will get Steem Power as a curation reward after 7 days from the upvoted thing (blog post or comment) is posted.
The lower your voting power (effectiveness), the lower your upvote is worth.
If you are upvoting daily on a regular basis, 10 upvotes per day is recommended to keep your voting power (effectiveness) around 80-90%. Sometimes I vote much more! :)
A lot of people are not upvoting the contents on Steemit, because they are confused by these two things!
They believe that their upvote costs Steem Power, but it isn't! Voting doesn't cost Steem Power!
Voting costs voting power, which is decreasing by every vote, but also increasing automatically by 20% every day, so you basically have infinite voting power, if you use it wisely! You just have to wait to recharge, but it doesn't cost you anything (except time of course, but not Steem Power)!
I hope this helps!

Thank you for reading!

Enjoy Steemit!
Have a nice day and have a nice weekend!