Hm. OK, well, falling apart and feeling sorry for yourself does not solve problems, even if you are doing it because it has always worked for you before. It's time for some rational thinking. Maybe you'll find that the sun shines if you act more like you are in charge of your life.
If you got pregnant around June 18 (or to add five days for the long life of sperm, June 18-23), you would have been told on August 24 that you were due March 11 (to 15), not April 1.
You say "I 'know' ultrasounds aren’t always accurate." But it looks like you do not also know that in your eighth week, the margin for error (when using an ultrasound to try to determine conception), is only perhaps +/- 2 or 3 days, not 2 or more weeks. So, please stop catastrophizing, and think clearly. While it is true that by the very end of pregnancy, ultrasounds can have up to a 3-week margin for error (again, this is by the 40th week), they do NOT have a noticeable margin for error in the eighth week. Not even in your 12th week, when the margin for error is only +/- 7 days.
You are also being inconsistent in saying you don't think the ultrasound in your 8th week could be accurate, yet falling to pieces over your interpretation of a 3D ultrasound as proof positive of your irrational fears. (Which, if you have had a chance to see a lot of 3D ultrasounds, you must know will not look that much like the future kid to come out of the pregnancy.) It's illogical to totally disregard one ultrasound as medically worthless while at the same time using (your perception of) the other ultrasound to disconnect from the baby you are carrying, weep, and carry on. You have no realistic medical evidence that the other guy is the dad.
So, why did you choose to take the 3D ultrasound as proof positive that the baby is not from your husband, when you prefer to believe the 8th week ultrasound cannot be right? More than likely, you're doing transferrence, something we see a lot in this community. Women who feel guilty can get sort of emotionally delusional, sticking their guilt and anxiety onto something that seems easier to focus on than the real problem. I'll post something here that I often post, it might not fit your situation totally but it seems to fit to some extent.
Every woman having this problem (not believing the dates, or the early ultrasound, or the doctor, or the DNA test -- yes, women do look right at their test results from top labs and still refuse to believe them--) has a different reason she is anxious. It could be guilt over her behavior. It could be catastrophizing, such as, they just know their whole life is going to end and the world will explode. (Which, when you think of it, even if their worst fears were realized, they would find a way to go on with their life.) The anxiety could be from carried-over problems from their marriage (possibly whatever caused them to cheat in the first place). It could be trouble accepting that they did something that in retrospect they can see is the opposite of their self-image, and fighting to maintain their saintly self-image in their mind by obsessing over something else. It could be worry about money. It could be wishing the other guy was the dad and being ashamed of that. It could be feeling God is going to punish them. Or it could be something else entirely. Those worries are not easy to control, especially if they are unspoken and buried. If the problem is shame or guilt, having cheated can't be taken back and the person just has to live with the fact that she did it. (This can cause a special kind of lingering angst if a women has never had to face up to having done something wrong that can't be taken back, glossed, or fixed.)
When such a feeling drives anxiety, the brain hates it that it doesn't feel solvable. A person's mind in this kind of existential stress often lets the anxiety settle on something more cut-and-dried that to the brain feels more controllable. (Like obsessing over the dates and what if the father is someone else, what-if what-if what-if.) The only way to fix obsessive wrongheaded thinking is to get grounded and face whatever it is that is really bothering you. One good way to do that is to discuss your real fears with a counselor. Not "I am ignoring medical evidence in order to obsess over the baby being from the other guy," that is not your real fear. Name the real problem and the fear will begin to go away.
It sounds like you are in the muck emotionally and are letting it settle on the face in the 3D ultrasound. This is a poor idea. If I had tried to decide who my son's father was by my son's 3D ultrasound, I would have to say his father was the Geico Caveman, he looked like nobody in the family on either side. But since there was no way the baby came from anyone else, I just thought it was interesting and neat that we could see his face as it had developed so far, and waited until he was born to see what he really looked like. You, on the other hand, are moaning about how "broken" you felt when you saw the picture. I'm thinking it wasn't the picture, it was probably that you were already feeling that way, and the picture just opened a steam vent and gave you a safer-feeling place to put out some of your dismay. Kind of the 3D-ultrasound equivalent of women who look right at a prenatal paternity test from Ravgen that says their husband is the father and their fling is not, and think "What if the test is ... WRONG?!?!?!" Don't let your fears make you look right at reality and ignore it.
Your dates don't suggest anything but that the dad is your husband. Your refusal to be realistic is problematic enough, but especially you are doing harm by closing your heart to this child. Even if you seriously think there is a doubt about paternity, you must at least not have any doubts about maternity. In other words, this is your child. You're letting your desire to self-flagellate over the affair get in the way of the love you naturally have for your baby. This baby has done nothing wrong, and is fully deserving of all your love simply by virtue of existing. I suggest strongly that you try to identify the source of your fears (ps - I bet you know in your heart that the 8th week ultrasound was correct) and talk to the counselor about them. The more you talk about your deeper fears, the more your fears will begin to leave.
Plant your feet and enjoy your life, as messy as it feels at this moment, because you are the lucky one.