Gem, it is a terrible blow when an ex, even one we left, one we know isn't right for us, finds a new partner. I was the same when my ex-h started dating. I look back in amazement and shame at how I reacted, and I was the one who left him. I can promise you this is the worst bit, and it won't hurt as badly in the future, whatever comes along.
Try not to project your ideas into what others are going through or thinking. What you see or think you see is just from the outside. There may be all sorts of stuff happening that you have no knowledge of.
Guessing can be distressing for you, inaccurate, and can influence your behaviour. Instead of thinking she's getting the best of him etc. etc. say, 'no-one can know what goes on in a relationship and trying to guess is just distressing me, so I will stop'. OK, it doesn't work right away, but it's a start at changing your self-talk.
You wrote '7 weeks of progress down the drain in an instant. I feel like I'm never getting over this, never getting over him.'
This is drastic, black and white self-talk. Replace it with:
'I've had a set-back, and I may have more in the future, but nothing is wasted. I will be back on track soon.'
'I not only will get over him, but I've already started. I feel like it's too slow, but it's natural to mourn, and I will gradually make progress.'
The above is a 101 on cognitive behaviour therapy, about replacing negative self-talk with more realistic ways of seeing the world. Ask your therapist about it, and google it if you're interested. It's very helpful in keeping yourself on a realistic level while understanding that the pain is natural and eventually healing.