I usually do well in both ranked and normals as a Bronze, but recently I've just been on a losing streak. I main support and usually go Leona, and actually won seven games in a row in my placements this season. Now, though.. I feel like I've actually improved since the start of the season, but for some reason I'm playing worse now and I can't stand to see my win rate gradually drop. .-. Here's my LoLKing: http://www.lolking.net/summoner/na/44479505
Does anyone have any tips?
Take a break. If you're getting emotional and frustrated then you aren't actually paying attention and your play will suffer for it.
Also, nobody cares about your winrate but you, and I highly suggest you STOP caring about it. You'll never be able to improve if you're too busy kicking yourself over a number that means nothing. You need to look at what you are doing wrong objectively, especially when playing a champion like Leona who is super high risk/high reward. Seriously, her only option is to go all-in, so you need to learn when you CAN and when you CAN'T, the only way to learn that is by falling flat on your face, figuring out why you failed, and then trying again.
Do ARAM or a different game for fun. Come back to the rift feeling refreshed
Without seeing any gameplay videos you look like you're playing very well: warding lots, building situationally, and not feeding.
So, I suspect that there may be a bit of a gameplay issue. Something I noticed is that you lose a lot to Leona's counters (Sona and Thresh, for example). Although they are considered counters, you can outplay Leona's soft counters, especially in the lower ranks. (Janna, Lulu, and Morg are Leo's only hard counters, in my opinion, which you should play safe against and wait for ganks). When I'm struggling against certain champions, I watch lots of gameplay videos on YouTube in hopes of getting tips about those champions' weaknesses or the playstyle to adopt.
Other than that, to reduce tilt I like to play music softly while I play (and usually not very sad-sounding music). Overall, echoing the other comments, do not worry about losing games because you should be playing to have fun! (gbay99's early videos helped me realize that mentality in case you are interested). So, glhf! :)
Tips on getting off tilt:
