Not sure if you are fixing your broken chain with a chain tool? Most modern chains are flush pin and are not designed to be put back together with a chain tool. You likely need to buy another chain or add quick links (not recommended). You can try to use a chain tool and sometimes get away with it if you’re very precise when setting the pin. It could also be breaking If the link you’re attaching is not loosened after it’s pushed through. Leaving the link stiff like that could explain your breakage problem. Once your chain is broken and put back together w a chain tool the horizontal flex created in your highest (and lowest) gears might be enough to break the chain.