Constantly feeling anxious and insecure about my relationship.

I started dating my best friend a few months ago and I feel like he doesn't want me anymore. His replies are dull and extremely late, we haven't seen each other in a while and he has made no effort either. Even though I made an effort towards it, he said he has prior engagements and that he's really caught up because he's starting his first corporate job. I'm left feeling unwanted, unloved and as though I might be bothering him and forcing him to give me his affection even though he doesn't want to. I have told him three time in 3 months of dating about how important it is for me that we talk regularly, everytime he says he'll do better but spirals back into replying very late. Everytime he texts late, I get extremely insecure and anxious about the relationship. I try to act like I don't care but I do care a lot and it really hurts me. However when he does reply back later, I feel slightly better and try to rationalise in my head why he might have replied late. I feel like this is really exhausting me and I want to know what to do about it.