@Camdengirl - I feel for you having to cope with so many stressful emotions, heavy responsibilities, difficult relationships and family problems. I find myself a bit curious and with a lot of questions, most of which I would hazard a guess that you have already asked yourself.
You say your marriage is difficult because your husband isn't affectionate or intimate, disregards your feelings and despite you trying over the years nothing much has changed. By this would I be right in thinking you have made attempts to talk to your husband, to convey your feelings, and maybe hit a brick wall? Do you have any idea as to whether your husband sees hope for the future of your marriage, whether he is content with the way things are or is as miserable as you? Would he want to work to improve things, change things or do you think he might be relieved, happy, agreeable to going your separate ways, even divorcing? If crunch came to crunch is there a hope in hell your husband would be prepared to leave the marital home to enable you and your granddaughter to remain there, to not unsettle a child that sounds as though she had a tough start in life.
You say your granddaughter lives with you full-time but don't say whether you own your home or are renting. If you own the property you should be entitled to half of it's value. If you rent this is obviously irrelevant but if you moved out you would need to be able to rent somewhere big enough for you and your granddaughter. If your husband left the marital home would you be OK living there so your granddaughter could stay in the only home she knows?
Does you granddaughter's disability require any special equipment, home accessories etc - if so is your current home suited to her needs? Would this need to be a consideration if you lived elsewhere, rented another property, moved in with your potential lover? Does your granddaughter have any contact with her mother, or father? Did your daughter's personality disorder, psychosis and/or drug use contribute to your granddaughter being disabled? Has she been disabled from birth or is it due to a later illness or injury? Are her needs likely to change or increase as she gets older? Could she improve, be independent later?
You say cash shortages impact any decisions you are thinking about making. Do you currently receive any state benefits? Are you entitled to any benefits? Do you get child Benefit for your granddaughter? If she is disabled you might be entitled to disability benefits, mobility allowances, carers allowances etc (not sure about PIP - someone on here might know more). Some benefits can be backdated - again we may have a Gransnetter with more detailed knowledge.
Is there no hope for your marriage? Do you and your husband communicate at all? He may be a bit selfish, thoughtless, inconsiderate, neglectful etc but is he ever abusive or is it just more of a stalemate situation? Does he help care for your granddaughter? Love her? Care about the impact a troubled marriage might have on her?
Have you had any in-depth conversations with your potential lover? Discussed possible future outcomes? Your happiness, mutual happiness, your granddaughter's happiness and future - along with any requirements, allowances, special needs she may have including schooling/education? Would he be willing AND able to accommodate you both? Financially support you both? How real, how strong are your feelings? Could any obstacles be overcome if you were willing and determined enough? Is it a case of rose-tinted glasses/the grass is greener on the other side?
You say that your "daughter is pregnant again & the whole sorry tale is starting again". Do we presume she will continue with the pregnancy? Will she want to keep this baby? Would the authorities allow her to keep a baby? Would you be expected to have custody of any other children your daughter has? Could you accommodate another child? When you say you "can't do it again" do you mean taking on another child? Potentially disabled child? Or the whole shebang of having to have contact with your daughter, social services, family court, solicitors etc - anything or anyone taking custody and responsibility for a child involves.
You deserve happiness. Can you believe you could carve out a new future for yourself? Trust yourself to love again, commit again? Can you see yourself 'starting over again'? Can you make time to go somewhere like Citizens Advice to talk things through and see where you stand? Do you get any private time/space at home (or even away from the home) so that you can seriously shut off from everything else and try to focus on what you want, how you could achieve this, what is realistic and practical and what is 'the dream'
I hope that you are able to answer some of these type of questions and formulate a plan for you future that works for you and can bring you some long-awaited happiness. Whatever choices you make I wish you luck. Gransnet is generally a supportive forum if you need us to discuss anything else, to update, to ask questions, to sound off or just get a little moral support. 
... My GD is with me because my daughter has a personality disorder & has been psychotic 5 times in 5 years (mainly due to drugs) including a spell in a psychiatric hospital. It has been a nightmare. I was going to leave 5 years ago then she got pregnant & the whole world went crazy. My GD is disabled so has just settled into school so things should be getting easier but my daughter is pregnant again & the whole sorry tale is starting again. I can't do it again. I'm 55 & I'm sure about the other man & really feel it's my last chance. I just need to be brave. Thank you all for listening.