Help!

I've been trying to get pregnant for 3 years now with no joy. It's really getting me down and feel as if it is all I think about! I've never had a regular period and last month was the first one I've had in almost 4 years! Had another this month (both very light but I'm taking it!) but again, lasted 3 or 4 days like the first one. My fiancé and I have been together 8 years and he has a 10 year old son from a previous relationship so it's making me think the issue is with me. I have had a miscarriage in the past when I was 17 but that was 11 year ago now so I know my body worked at some point lol I take ovulation tests quite often but they always come back negative so I believe I am not ovulating at all. I'm a size 16 so I'm aware I need to lose some weight but have pcos and find it hard to. I also suffer from horrendous migraines so whenever I exercise I end up with a terrible headache. Any suggestions/help from you guys would be greatly appreciated. Thanks 😊 xx