Longterm SO Yet No Sex in 8 years

I’ve been with my SO for 12 years. We’ve had our ups and downs, break ups and makeups but always came together, in love and committed to each other.

For the last 8 years he hasn’t wanted to have sex. It started suddenly after an intimate session together, then has drawn out for the last 8 years. I can’t believe how I even accepted this or allowed it or stayed I the relationship! We are now making marriage plans, ring to soon come and no sex yet. He has explained it’s a difficult thing for him after putting weight on his belly and being depressed here and there - but 8 years?! I’ve told him I will not marry him If this issue isn’t resolved and he has made efforts in the last few years by being more intimate with kisses, caresses and so on- so I’ve seen a change to be fair. However, no sex at all! I am beyond frustrated and very angry at myself and at him to the point where it all seems impossible!!! I feel ashamed for staying in the relationship and accepting this.

Trying to figure out what to do. We love each other and he is the man I want to have children with.