I am unable to have relationship

[Gimpee]

After bad marriage and a partner who left me when he found someone else I don't have the ability to love anymore

Yes, you do. You can love yourself. And yes, I know that sounds cheesy and like bollocks, but it’s actually true. The decisions other people make? That is on them, it is not about you. If you love you? Life will be way better, for you. No one else required.

I don't love myself guess that's why I don't believe anyone else could

So, that’s where you start. You learn to love yourself. Until you do, you can’t love anyone else anyway. So start with you. You have massive value, just as you are.

The best gift I gave myself was promising myself that I wouldn't have a relationship for 12 months. Honestly, it made a huge difference to my mind. It's ok to have a man, but I dont' need one. Try it.

Been on own for 15 years I don't think I will ever mend

You have to put the work in, all that time and effort you would have put into a relationship with your partner you need to make same effort with yourself.
Self love, self care is the best gift you can ever give yourself. You show other people how you expect to be treated because your standards are much higher.
Fall in love with yourself and your own way of life , you will never look back.
Relationships come and go, but if you can learn to live happily on your own terms it won't matter who's with you and who isn't.
There is nothing in the world so powerful or attractive as a woman happily capable of standing on her own two feet, not seeking external validation, living a full and interesting life. it is a freedom unlike no other.

I have two grown up sons who have mental health issues as I didn't protect them from the mental abuse of their father I can not forgive myself for letting my sons down

You don’t necessarily need to forgive yourself or anyone else to move on.
It happened. Don’t overplay the past. Only you can decide that you mentally leave it behind now. People on this earth have made it through absolute horrors and gone on to heal from it and like a happy life, loving themselves and others and being loved.
Why shouldn’t you. It is a choice.

Maybe some counselling/ therapy would be helpful to you, if you didn’t already have some. If you manage to forgive yourself for the past mistakes, you will eventually learn how to love yourself