Hello, thank you for the lengthy explanation for what happened. As this one is complete with the question, I will reply here.
In my opinion, it sounds like this man may have used you a bit for sex and companionship early on in your relationship based on the fact that he gave you an STD (meaning obviously he was with other women at the time) and also that you continued to doubt his intentions because he did not make them clear. Unfortunately, I cannot read his mind so I am not sure if these initial feelings developed into something more serious but it is a possibility based on the amount of time he was choosing to spend with you. The problem here is that it still seems unclear to you both whether or not there was the capacity for a lasting and exclusive relationship because neither of you brought it up. It does sound clear that he cared a lot about you based on that span when he was spending time with you but it is a bit of a red flag that your encounters were very patterned and involved a routine of gym, alone time and ultimately, sex. Typically, relationships that are bound for permanence have a bit of a broader range to them and at the least, there is acknowledged/spoken/exchanged commitment.
With that being said, I strongly encourage you to go and communicate your feelings for this man, especially since he is your neighbor and you will continue to see him. I know it will be difficult and take a lot of courage, but communication in relationships is virtually fundamental for them to succeed. This will allow you to open the chance to re-explore with him if there is potential, especially if you tell him that you really thought he may be the person you could spend the rest of your life with. It will also allow him the chance to tell you his true intentions behind your relationship to clarify if he felt he wanted to be exclusive with you, or whether he is just the type of man that is opportunistic and enjoys many women in his life. Until you communicate openly and honestly with him and ask the same out of him, this will continue to linger and drive you crazy analyzing and reanalyzing his behavior and feelings both then and now.
Relationships can be so tricky and confusing when they are blurry, and your history likely makes you more hesitant and distrustful as is. You definitely deserve a man who will adore and honor you exclusively, if that is what you want, but you also deserve clarity and explanation from this man about how he was feeling. And the only way to get that clarity will be to be super brave and ask. It is a given what if you don't open yourself up to him and the possibilities, then there will be none.
I hope this helps. Sincerely,