Yasuo has a larger chance of dying to Diana early on than Ahri does. Ahri is virtually never going to be in range of anything other than your Q until either one of you gets your ult, so your damage over time is going to be fairly low an inefficient. Yasuo has to get a lot closer to farm and trade, so he's much more exposed and more likely to be punished for aggression. Don't feel bad about your teammate getting a kill so early against Yasuo, compared to your matchup against Ahri; those fights simply aren't going to go the same way.
Diana isn't going to do much against Ahri until 6. You're playing a waiting game while trying not to fall too behind in cs or health, and you don't have too much of a reason to expend mana. For the first three or four levels, don't bother casting Q except to keep your mana from capping unless you know you're going to have to back before you can attempt to kill her. Wait until the health spikes from levels start to matter less and come less frequently, and when you're closer to having actual engage against her. Level 6 is your first big opportunity. If you're taking a lot of damage, ask your jungler to take over your lane for a bit (free xp and gold for him, so it's actually beneficial) to heal up and get some AP. When you get into lane, just be mindful of the XP difference and know she might level up mid fight, and be ready to all in her when things are equal or in your favor.