This is very hard on you. I don't think you are being selfish at all. But you do need to talk with your wife about possibly opening things up, and it might take longer to resolve than you think. I'm going to give a perspective purely from YOUR side as lots of posters have weighed in saying that for your wife's best interests you should invest all your energy into the sex life at home, limited though it is.
If you open the marriage, with her knowledge, without doing the emotional work you'll nees to do, it makes her feel more unattractive and worried and unsure of you. Then that will wreck your marriage in other ways and the sex (that you might be getting, somewhat transactionally, and in a very boundaried way) will not make you happy anyway because your life will be more tense.
If you both do the emotional work on being open, great. But can she take that on right now? It is a lot to ask if she's recovering, and it may not be what she is able to process. But if she's really uninterested in sex at all over the long term, this might be something she comes to as she loves you and wants your marriage.
Third option is you saying to yourself look, I don't want to put this on her now, I'll find a discreet fuck buddy and keep myself compartmentalised. That honestly could work and you will likely still be a caring and attentive husband. But it's a bit short term and you will likely feel shit in other ways and it will on some level corrode your closeness with your wife. Also it's worth doing some checking with yourself- you may think you want the escapist release of sex, but is it also that you want to be seen, known, nurtured and cared for, after such a long time as a carer (twice with your previous wife too?!). That is a powerful need and it leaves you vulnerable to other love affairs , so you may need to think about how to handle that.
So I'm afraid no easy answers. I'm really sorry for the situation you are both in.