How can I help improve the relationship between my guy best bud and my girlfriend, who are not really getting along for some reasons?

My bestfriend and my girlfriend are not really getting along for some reasons

I tried everything to make them get along, I always include/invite my gf if meand my bestfriend hangs out in the house or whatever..

My bestfriend was there since she arrived in my life and of course I love them both I dont want to choose.