What you say about certain shows/films being difficult to revisit, I know that. You'll find a lot of people have that same kind of relationship with media that is highly tied into someone they were close to but are no longer. I only got int GoT because of my ex, and we even abstained from watching Season 6 until I went out to visit her and we could chain it. So, I'm not looking forward to when Season 7 drops.
I'm deeply familiar with everything you're saying, and by hell it reminds me a lot of myself. Negative comparisons are a horrible thing to do as you'll always find someone that appears to have their shit together, or they're better looking than you/more talented/smarter/younger/combinations of all of these. I still do it myself, in fact right now the pervading thought in my mind is of the dudes that my ex knows, how they're musical (I can't play a thing), are in better shape (but I don't actually care about attaining that kind of body), have travelled more (I do enough of it myself) or are younger (you can't set a time limit on these things). It's not helpful, but I know what the general response to having someone tell you that is; I don't know how to stop. And sadly, fucking frustratingly, it's just something you need to figure out in your own way.
I don't want it to look like I'm providing guaranteed advice, as I'm still a big ole' fuckup in my own eyes, but you can take a step back and try some basic things to help yourself out.
The immediate thing I want to say is, you're only 19? Buddy, you have got so many years to change your mind and decide who you want to be, when I've been down my Dad says to me that he still doesn't know what he wants to do in his life, and he's in his 50's. Those people you see that appear to be so much more than you, that only works for them. You can't copy someone else's lifestyle and hope for the same results, you have to try and find what works for you. Some lucky bastards figure that out early on, others take much longer. I'm still a mess in this sense, but recently I've taken a few steps towards doing some fun stuff that I'm really looking forward to, and for the first time since around July last year I really feel like I have something to live for.
I understand how something like a cleft lip can impede your confidence. I can't say I have been affected by anything similar, but I did have confidence issues from a fairly noticable mole on my left cheek and I happened to start secondary school when Austin Powers: Goldmember released and all those fucking mole jokes followed me for what felt like forever. There's nothing wrong with quitting your degree if you truly feel that's what you want to do, but I would advise taking the time to talk to people at your university. I don't know what one you attend but I presume your UK based as you mentioned sixth form, but your university should have services available for you to be able to talk about how you are coping with university life. Take advantage of these services, and I would definitely recommend speaking to your GP and see if there's anything that can be done there.
There are people you can talk to, and I truly believe it's an important step to talk to people. I'm 26 myself, and I often feel jealous of people younger than me that appear to have it all together (that includes my ex) and quite frankly I'm a bit jealous that you've got seven years on me and have reached the point I hit nearer my mid-20's. I'd love if I could have reached those conclusions at that age, but it's not the way it works. It's a positive sign that you can actively recognise and identify traits you feel are negative and want to address, you just need to work on somehow making that next step of dealing with them, and it needs to come from within.
You're 19. You have so much potential you're not even aware of. There are numerous things you could apply yourself to, hobbies to try, interests to explore and more. It's a fucking hard job getting past those deeply rooted insecurities, and they will likely flare up over the years, but you can do it.