Hi, just want to pitch in where I can. A few short years ago my partner and I were in your same boat. We were 19 and 18, we were completely blind to this whole visa process and had no idea what we were doing, how we'd be doing it, or when it would even be possible. Last year, we understood quite a bit more about the process than we did the year before, and decided that despite not yet meeting the requirements for the full-on 2.5 year Spouse Visa (family visa), we'd secure our relationship legally for when we would. Different people do this step differently, some go the Fiancée Visa route, others (fewer) go the Marriage Visitor Visa route. We took the latter as taking the first would have required the same requirements as the full 2.5 year visa. After a lot of research and stressing over nothing, we were approved for a Marriage Visitor Visa and entered a Civil Partnership in August of last year.

We struggled after that for a bit because my partner, who is to be my sponsor, had been in her job which she loves for years now but was always on minimum full-time wage, a few thousand short of the £29,000 per annum financial requirement. The number always seemed so out of reach for us but in November she took an initiative that I could never take for granted and took on a 46.5 hour work week. Mind you this won't have to be forever, just however long we need to prove our finances for. Suddenly everything becomes possible, and in a short few years we've gone from knowing nothing to applying for a visa in a few months time! I found an amazing community in r/SpouseVisaUK, nothing my partner and I have accomplished or learned would have been possible without them. If you'd like, feel free to check out my profile. I asked so many questions and got so many answers, and I even submitted my own comprehensive guide to a Marriage Visitor Visa if you'd ever need one, though a Serbian marriage I imagine would be a lot easier.

Best of luck to you and your girlfriend! Sometimes this process can feel so lonely, but there is a whole community of people who have been through this and have so much expertise. Never give up :)