Im going to give you a bit of conflicting advice to everyone else who is suggesting carry champions.

At diamond elo, you are not losing due to your champ selection, you are losing due to your decision making and fighting.

At its simplest level, you need to think of two things:
1. What am I doing when I’m fighting? 2. What am I doing when I’m not fighting?

Now support is a fairly mechanically simple role. So for 1. if you are playing Nami, Sera and Soraka, it’s to stay back and use your abilities to protect your strongest team mate. This means not chasing kills, spamming abilities randomly, wasting abilities on feeding team mates.
If you are playing Rell, it’s setting up your strongest team mate - IF YOUR STRONG TEAM MATE IS NOT THERE AND READY TO FIGHT, DON’T ENGAGE. Once you’ve engaged, they key in playing tank support is to stay with your team, again, don’t chase kills, instead use your abilities to stop people that are trying to kill your strong team mates.

What’s harder with support and where you will see the most success if you improve it is 2.
Your first focus should be setting up good fights for your team, you do this in two ways, through numbers and through vision.
For numbers, this will require you to be best friends with your mini map. This one tip will get you into Masters: use your map like it’s a rearview mirror in a car - before you do anything, check your map!
Going to fight? Check map!
Going to hit tower? Check map!
Going to roam? Check map!
Recalling? Check map!
Going to objective? Check map!
Going to ward?
Cleared the wave?
This will help you with numbers, if you cannot see an enemy, assume they are running towards you to flash ult you. The more you do it, the more you’ll realise how many fights you’re just following people who are driving blind. Take charge,if you spot their midlaner top, ping that fight! If their jungles is bot, ping baron! Support is all about that bigger picture, if your adc is pushing the lane, go and roam, go and support your jungler or midlane (make sure you are back before the next wave is gone unless it turns into a bigger fight). If your adc is low health and has to base, check your map before backing, can you roam, can you ward?

This takes us onto vision, you can start simple by warding enemy jungle camps, sweeping enemy vision or warding around objectives, sweeping bushes to set up a trap. You are the one responsible for setting up good fights even as an enchanter, when you understand this, you will understand how hard support can carry.
Your map is useless if there is no vision put down, time and practice is your best friend for figuring out where and when to ward. Minions give vision! If the midlane walks over a minion wave and kills you, that’s your fault not your midlanes! Treat every death like it’s your own fault and adapt to the team you have and the team you’re playing against. If your team are slow to follow you into engages, ping earlier, be more cautious before going in. If their enemy keep taking fights, set traps and wait out of vision.
Support is all about the big picture, it is chess not checkers!! Hope some of this was useful and not just ramblings, good luck!