I think you've already identified yourself what and why you're seeking it.

I think the reason that you probably sexualise your surrogate mother figures is to try to make sense of your desires to your sub-conscious.

Part of you feel that what you're seeking isn't normal, and in todays society it isn't. Howver there is absolutely nothing wrong with it. A generation or so ago you'd probably have found the surrogate in your grandmother, and some people still do, but as families become more and more individualised it's just less feisable for this to happen naturally.

It's a really confusing time being a teenager or a young adult, and everybody wishes they had somebody there to offer them support, encouragement, and comfort when nessacary.

To an extent you'll probably feel it naturally largely subsiding as you become more emotionally and financially stable and are nolonger seriously missing the emotional support that you don't currently have.

The comfort, and support you desire could easily be fulfilled with a relationship of either sex if you find the right person