Why do I keep putting myself through situationships?

By situationship, I am thinking that you mean it is a non-platonic relationship between you and the guy, but it never fully develops into a serious or formal relationship. It sounds like this status quo is difficult for you, perhaps as you never know where you stand, and this is causing you anxiety and overthinking. You say that you believe you may be self-sabotaging - the context around this would need to be explored to fully understand the ways and reasons you may be self-sabotaging.

Although it would seem counter-intuitive to sabotage a relationship or a situationship, because part of being a human being is to find connections with other people. However, with making deep and intimate connections with others, comes the risk of causing ourselves pain should the relationship not work out. The instinct to keep ourselves safe from this risk of pain can take over and this is where we begin to self-sabotage. 

It sounds like this situation may have happened several times over - it can be a hard habit to break as the "reward" of sabotaging is that we avoid intense pain. Although perhaps you never "upgrade" from a situationship to a relationship, which in turn causes some tough feelings, it still enables you to perhaps gain the positives of a relationship without the commitment and fully investing yourself in that person, which could potentially leave you open to having your heart totally broken, should you fully invest your heart and soul. 

I'm wondering what your past experiences of relationships are - have you been hurt, disappointed in some way, cheated on? These experiences might subconsciously trigger your desire to shield yourself and self-sabotage. Self-sabotage can show up as wanting to prove ourselves right - in many of your situationships you have seen them not turning in to the relationship you desire. You imply in your message that you push people away through overthinking - how does this present outwardly? What is it about the guy (or yourself) you are overthinking and what is behind your overthinking? These are things that would need to be explored to understand the full context of what is going on.