In love with someone else, how to fall in love again with my partner

I might be in love with someone other than my partner. I really don't want to, but he just makes me feel a certain way. That first time spark, the excitement, etc. We're very like minded, something my partner and I are absolutely not.

However, I know that it's not worth ruining my current relationship over. I'm not entirely certain about a future with my partner, as we have our problems, but I love him and don't want to lose, nor hurt him.

The ideal thing would be for me to keep my distance from my 'crush', but we work together. Quite closely actually, and I really can't avoid him.

It also doesn't help that his behaviour is confusing. He says 'nothing will ever happen' and 'he doesn't want to come between me and my partner', as well as that he doesn't seem to be over his last break up. On the other hand however, he flirts with me constantly. He says it's just banter (and my way of just joking around is also often through flirting), but the way he flirts is not what I'd consider just friendly. I just don't buy it.

I want to believe that he's truly not interested and respect his 'no', but I don't feel like he's being completely honest. On the other hand, it doesn't matter. I don't know him that well in the first place, and there's no future between us. I don't want anything to happen anyways, regardless of how he feels, because I don't want to ruin my relationship. But I can't stop thinking about him and even though I really try, I just can't get over him.

It feels so unfair to my partner because he's been amazing the past few weeks, but I don't know what to do.

How do I fall out of love with my colleague, and in love again with my partner?