I have 6 capital ships (all above level 6) and 50 frigates/crusiers.
This fleet is very small. You have lots of capital ships, but relatively few frigates and cruisers, and that's going to seriously affect your damage output and overall staying power. Capital ships offer lots of interesting special abilities, but in terms of brute force are somewhat underwhelming.
i have reserched all of the tech (im guessing they have too). i can not even afford to replace the ships they destroyed...
Then you have probably lost. The AI is a very poor attacker and high-level TEC capital ships can be particularly nasty, so I'd imagine you could successfully defend for hours on end. However, as TEC in the late-game you really need to be able to build and sustain large fleets to go on the offensive. It's probably still winnable in theory, but as others have mentioned it's going to be time-consuming and impractical by this point.
Try to focus more on expanding your empire and fleet. Technology is effectively multiplied over your entire empire, so it's a relatively small effect for small empires and a massive effect for bigger empires. To facilitate technology, you need to focus on early expansion, and that means developing your military early to secure new worlds. In terms of colonization, try to capture one new planet every 5 minutes (even faster if you can manage it). Bigger empires can have bigger incomes and support more structures, which enables you to support bigger fleets. Whether it's +10% damage or resource income, if your empire and fleet are bigger you'll get more benefit out of it.