@Elinxs27
I really hear how afraid you are about possibly losing this relationship, and it's completely understandable. It sounds like you truly cherish having him in your life, but do not know what to do now when you both have hit a rough spot. If you really do not want to lose him, however, and if he is on the same page, then both of you would need to properly sit down and discuss ways of making this relationship work. It takes two to build a strong bond. There is no way to tell if someone is falling out of love with you for sure unless you straight up ask the person. Don't let your insecurities and irrational fears get the better of you. It's very easy to jump to conclusions and assume things, but until he says so himself, the best that you can do right now is to show him that working this relationship out is worth it despite the fights recently. I like to ask this to my partner when things feel like it's going downhill, do you think that in summary, we have more happy memories or more sad memories? And from there, you can get a feel of how good/healthy your relationship is. If the answer is: "sad memories more recently", well perhaps there are some things that you and your significant other need to talk and sort out.
All in all, I know it's easy to have your emotions take charge. Clearly, you seem to be feeling very scared and concern about breaking up if you dreamt of your partner getting with his bestfriend. What is very important though is that if you both really value each other, communicate and try to compromise and figure a common ground :) It's hard work, but all relationships are like that. Good luck!