I usually don’t suffer from being cold at night so I am not the best for advice, but I’ll tell you what I use in the alpine.
The North Face synthetic sleeping bag rated for 0C (comfort), -7 (limit), -26(extreme). These seem to be realistic ratings to me, but everybody sleeps differently. One thing that I feel is important is to make sure your sleeping bag is washed before your trip. A clean sleeping bag is warmer than a sweaty, dirty bag.
I sleep in socks, a merino t-shirt and a toque. The toque usually comes off. I use a sleeping pad with R-3.4 insulation value. I use a two person tipi tent with a bug screen inner and a ground sheet.
I think it’s important to make sure you are well fed and hydrated before bed. Make sure you piss before crawling in and try to eat with a bit of time before you crawl in so your body isn’t diverting blood and energy to digestion. have a cup of tea before bed.
some folks put a Nalgene filled with hot water in the sleeping bag with you. I’ve never used this before, but I think it’s a good idea.
Also, remember to relax. Nervous tension worrying about being cold can make it worse than it really is. Mind over matter. Maybe easier said than done… One last thing is it’s not supposed to be the most comfortable sleep you have ever had.
This is just what I do. It won’t necessarily work for everyone.