I need some suggestions

I'm trying to turn down someone but I don't want to make them suffer too much.

We've been friends for a long time and they are a very fragile person since they already suffered quite a lot in the past and used to rely mostly on me.

Sometimes I get the feeling that they are almost obsessed with me and tbh it makes me feel a bit uneasy.

Techincally, I already turned them down more than once but I don't think they have fully accepted the fact that I don't like them.

They are also a bit manipulative and not very honest and I am quite naive and sometimes I fall for some tricks and it annoys me. Because of this I am also getting tired of this person but also feeling guilty.

I am afraid that if I am too harsh with them they will do something dumb (since they already are depressed enough)

But if I sugarcoat my words they will not really give up and move on.

I'm not very sure what to do ;-;