1st time backpacker, feel kinda overwhelmed. Should I buy a different bag? Could I get some second opinions?

Here's the deal.

I'm going on a 3-month backpacking interrail trip around Europe in mid-July.

I bought a big ass 70l bag (Vango Contour 60+70) months ago, and now I'm kinda regretting it and wish I bought a 40l after doing more research and realising my mistake.

I cannot return the bag because I bought it months ago.

Should I buy a 40l or just stick with my 70?

All I'm packing is clothes.

Here's why I can't decide:

If I buy the 40l, I'll have to buy overhead cabin space too.

It's more expensive in the short run but possibly more comfortable and easy to handle in the long run.

Or if I stick with the 70l, I'll have to fuckin check the bag in cuz it's huge.

But I'll save a bit of money in the short term.

Plus have some extra space (I'm not overpacking I don't think).

Am I overthinking it?

Would the size difference make that big of a difference?

What would you do?