Get some screenshots dude, they can provide lots of information so we can tell you where is the problem... (F1 on keyboard, files saved in game folder in folder called Screenshots).
Until you get the screenshots, consider the following in your design:
When you start going big (aka 2.5m), use struts to tie stuff together.
Use fins on your rocket.
If you see yourself going beyond 100m/s in the first 1000 meter, then SLOW-DOWN, you need to go fast, but not very fast as your rocket will not hold against gravity force and aerodynamics.
Try to make gravity turns slowly and start turning slowly once you reach 50m/s. I do my turns starting from 50m/s and do them on 5 degrees every 5-10 seconds. just make sure you are doing them slow not to flip the whole rocket or blow it up.
You can use 1 core orange tank + a Mainsail, + 2 radially attached 1 orange + x200-32 tank on both sides with 2 Mainsails on each of them, activate the radial ones first, then the core once you go high in atmosphere. Those will give you the required lifting safely, but make sure you monitor the ascent speed not to go super fast while low in atmosphere.
Again screenshots are great
And welcome to the party!
SalehRam's post in I need help with building 2.5m rockets to the mun in 1.0.5 was marked as the answer