I find real life and the internet to be two different things when it comes to bits.
In real life, tom thumbs and their cousins (argentine snaffles, "training snaffles," etc.) are considered pretty mild bits and the horses like them really well. I ride my Mustang in something similar and probably have a collection of about 10 of these bits with different length shanks, different looks, etc. They are my favorite bit. If I could only own one bit to use on every horse I ride, this would be it.
However, on the internet, there seems to be this notion (mainly due to one article written by a trainer) that these are awful bits, are too strong, and confusing to the horse. THAT is EXACTLY what I read and my friend said it was like a bit with training wheels.
Well, all I know is, that I have only ever ridden one horse that didn't ride well in a tom thumb. I swear I can use it on just about any horse with great results. Of course I have light hands and ride with little contact. I'm not quite sure how lightly I ride but I do know the previous owner rode her lightly - not just in the mouth but saddled as well.
So I say if you have gentle hands and the horse does well in it, use what the horse goes well in.
(But, I know most of this forum will likely disagree with me). I feel like the "Defender of the Tom Thumb." I'm still learning how well she does. Today was only the 2nd time I've been on her since I bought her almost a month ago - due to the weather and a broken foot (long story!) She did well today but moreso turned her head toward the side I was gently pulling (and with the same leg pressure) than moved her body in that direction - so I am going to assume (yeah, I know) that I was not using the bit correctly. An important PS.
Your curb chain in attached in the wrong place. It should be fastened to the same rings that your headstall is attached. The big rings that you currently have your curb chain on are there in case you want to attach a rein as a snaffle with no leverage at all. It is not meant to be a curb chain attachment. I suppose this would have a large impact on the statement above? I will bring this up to my friend, because it is her bridle, not mine. But, it is good to know for my own benefit and if I use it, I will move it and let her use it as she wishes. Perhaps there is a reason? My friend used to show in barrels and reining, so I usually defer to her judgment on most things. But I am also aware that her knowledge may not be beneficial to me and my horse in terms of riding/learning.
I have a feeling the curb chain is either too low or completely ineffective in the place it is now.