I've always had shoulder length or longer, fine fragile hair and it's just gotten worse with age. The sides of my hair especially just would not grow due to breakage, then after a period of extreme stress the front got very very thin so my scalp was visible. Over the last few years I've tried all sorts. I was considering minoxil as a last resort, but wanted to try something/everything else first.
I think you basically have to hit it from all angles, take supplements with iron in, I take Florisene, like pp have said get your iron levels checked and ask for the figures, not just if you're in the normal range or not. I also take a vitamin D supplement.
Use a scalp spray, I've had good results with L'Oréal triple resist argenine spray, but it's been discontinued now! You can still get it online from a few places, it's quite cheap and I didn't think it would do anything. Initially I just used it after washing my hair, like 3x week, and didn't notice much change, I then started using it every day, morning and evening, and I'm quite shocked at the results, a definite improvement to my fringe area! Obviously you need to stick with it for at least 3 months before you're likely to see any improvement. I'm sure there are other scalp stimulating type products out there though too.
Keep the hair you have in tip top condition, once a week I sleep in Danial Galvin Jr organichead argan oil detox mask, once a fortnight also sleep in olaplex (separate night). I've also had good results with redken extreme length range, particularly the primer and mask. Also had good results with joico K Pak range.
I don't use a hairdryer at home at all now. I have T section highlights every 3 months, and that's the only time a hairdryer is pointed at my head.
My latest discovery is 'low' or 'no' foam shampoo. I've only been using this around a month but it's been a game changer for me! I can already see, and feel, a big improvement to the condition, especially the ends, feels much less 'bitty' and my sides have definitely had much less breakage. Since using this type of 'nopoo' I've been doing my conditioning routine prior to washing, just putting on damp hair and washing with 'nopoo' next morning.
If you don't get any joy I'd deffo go see a trichologist, I know how distressing it is and how hopeless it can feel when nothing seems to work. But keep trying OP, hopefully you will find something that makes a difference.