This is a common thing to feel after you hit a bet.

Of course whenever you win it doesn't feel like you bet enough. But you have to realise that there is some probability that the bet would not have hit, and you would have mitigated some losses due to you betting a smaller amount. This is especially true when it comes to parlays which are generally very -EV (as each leg is -EV and it is multiplicative).

Take the winnings and don't feel the need to chase the imaginary losses that you are keeping track of.