want to move on

This is my first posting, so please bear with me.I need some impartial and practical advice.
Im 52, as is ex. We co-habited( bought house together) for 7 years before marrying in 1990 and having first child( now 17), second is 14.Weeks after her birth he started an affair with a work collegue.I found out, and he moved out and in with her about a year later(her house.)He paid me about 1/3 of his income
Ex decided he had made a mistake, wanted to come back- and reader I let him. I went back to work full time as he had run up big debts, he also works and was earning well but spending well.
18 months later I discovered he was having an affair with another collegue- the one who got the previous girlfriends job!He moved out again, signed house over to me ( he wanted nothing to do with upkeep and hassle)and new girlfriend moved into his flat a year later. They are still living together in a house in her name.
Ex has never had the children overnight: he calls here 3 times a week, has a key, we spend every Xmas, etc together.
We get on well on one level- its been v. difficult for me but worth it for girls sake, but they are nearly grown.
Ex was made redundant earlier this year, and I then found out he was earning £70k+. He was paying me £1150 a month . I earn 35k.All this is informal. The girlfriend is selfemployed and earning a lot, their mortgage is not very large.
He is now working part time self employed, earning the same as me. He got a 60k lump sum, which he used to buy a new car and pay off credit cards.He has a good pension pot,( worth 32k at 60) not yet drawn on , and I am still the named person for spouses pension and benefits if he dies.( currently 14k/yr, this will increase with time/inflation etc )
Every time I have raised getting a divorce he has got really angry and I have backed off, partly to keep the friendship going and partly because Im concerned I could end up worse off.
It worries me that this is all informal and I really want to move on. My mortgage is nearly paid off, but the girls will need a home even when they are in college and I dont want to have to sell the house to survive on retirement ( I have a very small pension entitlement).
Can anyone advise my best option?

Have sent a PM