Pick a support that can do camps, like CM, etc. That will help you to get some farm while stacking.
Buy smoke to rotate and gank, if you can leave your safe lane core alone for a bit - if not, babysit the shit out of him.
Always try to get one of the runes, to get a gold/xp boost or some nice buff for ganks
Never rely on your team doing smart things, always assume they are stupid as fuck. Treat them like new born babies with zero perception.
If needed, build items that give you early dominance or help secure kills.
If necessary, get last hits on creeps/heroes if your core is too stupid to do so.
If your warding is good you should see enemy players. Study their behaviour and bait them to make mistakes. Punish these fools, they do not belong in your MMR. Punish them for every minor decisiont they make, until they cry or rage quit.
In general: as support you have high early-game potential, which you need to use in order to get the needed gold advantage to buy better items that will help transition well into mid-game, which will give you another good lead to survive transition into late-game. You shouldn't go stack as solo support only to stack. If you stack, also farm a few camps, then return to lane or gank. Stacking should be something to do inbetween walking from X to Y, especially as solo sup.
Other than that: if you feel your team can't carry well, try to pick a semi-support that can transition into a good semi-carry like Venge, etc. That way you can always buy dmg/crit items or something to help tank more dmg during fights.
If you really are willing to improve and win those games, post your dotabuff and ppl will discuss item builds with you. That's the least reddit can do - for more improvement, watch your replays and analyse your decisions/mistakes. Watch pro players (pick from dotabuff best player list for certain heroes, download matches, watch and learn) to see what they do differently.
And: practice, practice, practice!