Puppy walks perfectly without a leash, but is a disaster on a leash

Me and my partner have a 11 week old French Bulldog puppy. She is a sweet girl, but super stubborn and feisty.

The first weeks we had her she suffered from Giardia, so she pooped everywhere. Luckily ely this is over and she poops almost every time outside and also pees a lot of the times outside.

Here is the thing: When I walk with her without a leash (I only do this in the forest with almost no distraction), she is the sweetest girl. She will follow me loyally and does her pooping and peeing.

When I walk with her on a leash she is a disaster. She will not follow me, she wants to decide where we go and she will put EVERY THING in her mouth. What could be a 10 min. walk, turns into a 30 min. battle.

I try everything. Giving her treats with good behavior and give her a firm no and take whatever is in her mouth out.

She also not fully understands the command “come/here”.

I don’t know what to do. Somebody experienced the same behavior? Any advice?

I am feeling very negatively towards her right now and I absolutely do not want to feel this way.