Agree about the communication thing above - and keep this up. As her Dom, have her write a journal (or talk to you in person) of how she feels after each session with you and use this to modify your practise. Make sure she knows to make this an honest appraisal of your performance (no false praise but lots of constructive criticism). This allows you to benefit from her experience without losing the Dom position too much as rather than telling you what to do (which would be her topping from the bottom) she is instead suggesting ways you can improve based on what worked for her. Subtle difference but worth making.

A BDSM checklist is always worth a play with. Do a search for one online (there are a few). You both fill it in and compare answers and discuss them and make sure you research things you don't understand (and there will be things both of you don't understand, regardless of how experienced you are). From this you can get a list of her likes, dislikes and hard and soft limits and from that you can derive a list of rewards (activities she likes), punishments (activities she does not like but will still do), targets to aim for (soft limits) and things to avoid completely (hard limits).

You may well have to get over the 'I'm hurting someone I love' feeling... this can be hard (been there, done that, not spanked her hard enough because I thought I was hurting her too much, she called me a pussy for not spanking hard enough... ). Best way to get over this is to work slowly and build up your intensity each session until you find a point where she calls a stop (and make sure you have a safeword for her to do this with - I like the red, amber green system myself). You then have a good idea of her pain tolerance which you can work on each session.

Make a plan... work out every thing you want to do and in what order and even how long on each. Write the plan down. Even have a crib copy hidden somewhere in the room while you are playing (blindfold her so she can't see you reading it ) if you think you may forget. This is important because it lets you think through all activities you need to do and make sure you have the right kit, ready to use near to hand (to prevent delays while you search in drawers or have to pop out to the shops for stuff) and that you have thought through all the safety. You may not stick to your plan exactly (you may spend 20 minutes spanking instead of 10 or decide not to use the gag...) but it gives you a vague framework and allows you to think of issues like the fact that you can't insert a butt plug once you've tied the crotch rope (may sound silly but beleive me this can happen...). As you get more experience, you will need your plan less and less and may get to the point where you don't need to make one at all on paper - just think it through in your head - but it is a useful crutch in the early days of learning.