There's a third person in our relationship

[Tigerqueen]

I’d really love to hear everyone’s comments on this to see if I am over reacting. My partner (female) and I (also female) have been together for a year and a half. We see eachother maybe once midweek and at weekends. However, I feel like we have a third person in our relationship, her gay male best friend who she has known for many many years. Both he and I get along well so no issues there. BUT, they are together all the time. Every single day and every single night, and I mean every single night. He is at her house literally 30mins before she goes to bed. Whenever we chat about how she’s doing or what she’s up to she is always with him. She did say to me not long ago that when he is single he tends to do this and latch on. At the moment, he doesn’t have a job so has probably too much time on his hands. If my partner and I make plans to see eachother he needs to know what days so he can make plans too as she isn’t available for him. Recently she didn’t tell him until the last minute that she was staying at my house through just being busy and he was miffed because he didn’t have anything to do and had presumed they would be spending the weekend together. When we do see eachother he is constantly texting her. When we are on a date night or watching a film he’ll phone to talk to her, he even called her from a first date to tell her how it was going (in front of his date) I feel a little silly for being bothered by this but it just always feels like we have someone else in our relationship. Am I wrong to feel a bit off with this?
 

I’d also like to add that she is in the process of buying a new house and he wants to rent a room and live with her, I have no idea how I’ll feel when I visit her and he is actually there all the time, we’ll have no time to ourselves