Maths problem betting algorithm

I have a progression that i need help with. it is a simple problem where you add the amount per bet on loss and decrease a little when win. I have a couple small issues that i need fixing to come up with an algorithm because when you keep going down bet value on large win streaks you are betting and winning small amounts and when your bet rises large amount on lose streaks you are betting high amounts. The system I have now works really well in reducing the probability needed to make profit such as 50% chance games only needing 46% to win to profit. So just need a little fixing.