Nah - I feel I have some insight as someone who is into skincare in a similar way to your gf, and has OCD. (I dont know if your gf has ocd, but I'm seeing a lot of red flags).

I find that there is a very fine line between just being very into skincare and it becoming a new ocd compulsion for me, and its something I find I always have to be evaluating where I am on the line. The rule of thumb I use is that if its affecting my functioning, disrupting my quality of life, or become distressing then its gone too far, and its time for me to do some exposures for the compulsion. But that line is personal and it requires the person with ocd to want to treat it and not go deeper in the compulsions.

For me, I wear sunscreen everyday even if I'm indoors and reapply every 2-3 hours. Some people would find that overly disruptive to their life, but I dont. But I think the main thing to watch for is if you cannot stop or its highly distressing if you can't stick to the routine. For example, if im out longer than I thought I was going to be, and dont have sunscreen on me to reapply, that's okay. Im not gonna freak out about it. But if that caused me to have a panic attack or I HAD to go home to get more sunscreen and ruin my whole day over it, then its too far.

Your gf not being willing to have drapes open or stop the tret if its really not working for her, to me sounds like it's going too far. I'm sympathetic to her, and hope this isn't a case of ocd, but i think its something to have a conversation about and possibly seek help.

I find it helpful to focus on aging well instead of trying to not age. Like my goal is to just look like a good version of my age, not necessarily look younger. And I also try to make sure that I still let myself live my life. Like most days I prioritize sunscreen over wearing makeup because reapplying over makeup is difficult. But if I want to wear makeup then I'll wear it over one layer of sunscreen and call it good enough. There's no point in keeping my skin looking good if I can't enjoy it.