YANBU. Doesn't matter if she is not real (she might be). The emotional betrayal is huge. A friend of mine experienced this.
In her 60's her hubby fell for an online love rival half his age. What happened next was her hubby started sending the young lady money, to the tune of £15,000 of their very carefully nested savings (older retired couple with good savings and nest eggs).
My friend then went about taking all the necessary financial steps to cut his b.....alls off, starting with changing her Will to remove him and leave her bequests to her children only, and then changing her bank account etc. They are currently living "separate lives" within the same house but she is also taking steps to divorce him and leave ...... and present him with the divorce papers and news about the rearranged Will and finances all as part of her final showdown surprise !!!
Your other half may or may not have been drawn into financially supporting this cyber babe love rival (yet) ...... but his head is there obviously and his heart is not in the right place (clearly he doesn't give two figs or concern about his own biological children which he has with you).
It is easy to get sucked into an online cyber affair ...... when domestic life and kids mess and domestic duties are all a bit too boring (for him) ...... but a person does kind of actually have to go looking for it and having found his "cyber babe" online he has then willingly and voluntarily chosen to continue to pursue the affair (for months) to the point of declaring his love for her.
His head is in the clouds, his heart is not in the relationship with you.
Hit him where it hurts -- financially (!!!).
Throw him out with a bath towel and a few clothes and say good riddance.