I dated someone...Lets call them person X. X was a HORRIBLE kisser. I mean, HORRIBLE. It was like X attacked your face with their slobbery mouth and mauled you, then prodded you with their tongue forcefully. I think X read my resistance as playful, because the more I tried to pull away or change things up a little- X would slobber all over my face more. It was like being attacked by a soggy plunger. It was awful. Which was a shame because X themself thought they were a great kisser. Previous exes of their had told them when X asked that X was a good kisser...not so.
I tried to subtly hint at what I liked- When they plunged my face I would kiss slower, gentler, less tongue. But X was so sure that they were an excellent kisser that none of the signals I was sending out registered with them. Regardless I was telling X what it was I liked, and X was too busy mauling my face with drool and being overly confident that they totally messed things up. We broke up for unrelated reasons, but by that point...I dreaded kissing them and would have ended up breaking up with them anyway. Total shame because X was very good looking and I was otherwise totally attracted to them.
Now, not saying I am perfect at these things. As much as I hate to say it I met my current boyfriend drunk at a party. The thing about drinking is- It can be seriously hard to feel your lips and the pressure you are exerting. So while you are making out with this person you might be kissing WAYYY too hard. Or too rapidly and oddly another thing that happens when you can't properly feel what it is you are doing. I say thsi from experince because buised lips arn't fun for either party even when you don't realize you are doing it! Keep this in mind while drinking.
NEVER start a kiss open mouthed. Oh my gosh. Don't do this. EVER. Play a little bit, lead up to it. And then once you want ot use some tongue please don't force your whole tongue into my mouth and choke me with it. A playful touching of tongues is often enough.
Read your partners signals. If I start kissing slower, or softer, or harder, I am telling you with my body language what I like. If I touch you with my hands, that is saying something too...and now always good. A hand in the hair- often a sign im into things, a hand on your side can just as easily mean I am really into what you are doing, but a change in pressure, pushing lightly with it might mean STOP. If I move away I am saying less...Closer I am saying..nope! (shocker)Not more! I am saying just right. If I start changing the pace though, then maybe I am saying more. Which doesn't mean go from a soft, slow kiss to seizing the back of my head and driving your tongue down my throat or using your lips to gum my face 1000x a minute. LISTEN TO WHAT THEY ARE TRYING TO TELL YOU!
Don't drool on me, slober on me, make my face slick with your saliva. Moist, soft lips are nice. Wet lips are bad. Dry lips are not great to kiss too.
Kisisng a stranger- How about instead of diving into making out you have a few kisses, pull away slightly(not too far, don't make her feel weird about it..just half an inch from her lips even) and see if she kisses you again. You need to gauge her reaction to whats happening and not your own. Don't fondle her. Just because she let you kiss her doesn't mean she wants you grabbing her ass and grinding her aggressively.
You should have asked your ex what made you a bad kisser. When someone tells you something like that, it can hurt but it is a great way to learn and be better