IWTL how to not like people and have a less optimistic assumptions of peoples' actions

I generally assume the best of everyone. In my immediate life, no one is evil, even people I recognize to be social climbers. Everyone is fundamentally a person with complicated history and motivations, and I simply try to be diplomatic and understand them as best I can.

When people say or do things, I generally take them at face value. If they said they did X or believe Y, I usually believe them. I have no reason to think that they are lying unless there is evidence to the contrary. Now, obviously as I get to know people, I'm not lacking in emotional intelligence here -- I can discern underlying motivations, insecurities, etc. I'd like to think I'm good at reading people in the moment and relating to them.

However, I lend myself to being taken advantage of, because I have indoctrinated myself to put negative assumptions out of my mind. I always try to discount these feelings, where possible, and it's so ingrained that it's hard to break that programming. This positive assumption of people leads me to not using my noggin when I should.

For example: Did they really send that email? Are they actually sick or unable to do their job? Is this task hard? Are they actually a jerk or are they misunderstood?

Another example: I had a stalker that got so bad and I was harasssed for 6 months that I almost got a court order, which the judge simply said "you needed this months ago and we're upgrading it." Never went through for admin hurdle reasons (plus the harassment ceased). This stalker reached out to me again a year later and was trying to make amends, apologize, actually wanted to just be friends; I wisely listened to friends to not even respond, but the fact that I considered giving him the benefit of the doubt is a sign that I need to reevaluate my stance on people.

Basically, I want to be able to reorient myself to saying "I really don't like that person" and just not play nice. I feel like I end up being diplomatic and unless that person is overtly hostile to me, I'll always stomach my own negative feelings and just try to be objective. I may make fewer allies that way, but surely this state of affairs is not beneficial either.