You've already identified what keeps you stuck- your inner dialog ex: always convinced they have an ulterior motive or are not sincere"
As long as you believe that no man is honest, you'd remain experiencing the difficulties you've shared. Your thoughts dictate your feelings and your behavior. You can not do the same thing and expect a different result.
Otherwise, you are sabotaging each possible new relationship right from the start.
Monitor your thoughts- how often you think these things, what you feel and what you do. There is a way to change your thoughts and your expectations. Tools such as hypnotherapy, autogenic training, cognitive therapy are some of these options. It is all up to you what you want to try and how persistent you'd be in your attempt. The past does not have to repeat itself.
Hypnosis for Self-confidence and Self-esteem (Hypnotic Empowerment Series for Self-Awakening) (Hypnotic Empowerment Series for Self-Awakening) by Janet I. Decker (Audio CD )