Making out with your girlfriend can be serious, especially when you describe it as "A LOT."  That can mean that you are doing it very often and perhaps too intensely. 

 I am convinced that sexual activity should always express a relationship that exists and not be just making out.  I also believe that love means that you will respect each other and that you respect God's law about marriage and sexual activity.  If you do, then making out will not be the major part of your relationship.  (If it is the major part of your relationship, then slow down!)  Remember, if you are in love, then you can go slowly with making out, because you will be with each other for a long time.  Take time to build a friendship and build it slowly.  Things that last will grow slowly.  Love that blooms fast, often dies just as fast.  If you are not going to be friends with each other for a long time, then making out may be just a temporary pleasure and not an expression of a lasting love.  If you purposely do something serious (such as making out) and you know that it's not the right thing, then it could be a sin.