You need to go through processInstance records....   I have some code which I'll put here which may help... it's pulled out of a large VF controller which presented a list of Items for approval (Items to be Distributed to customers).  I put a bunch of checkboxes on the page next to the item information so the user could select which ones to approve.

So, this is a subset of the controller which, perhaps, will help.

First, define a helper class for the approvals on my Distributed_Items__c objects.

// Helper class to put an envelope around something for approval
	public class checkBoxEnv {
		public Boolean approve {get; set;}
		public Boolean deny {get; set;}
		public Distributed_Item__c distItem {get; set;}
		public List<ProcessInstance> process {get; set;}
		public List<ProcessInstanceHistory> stepOrWorkItem {get; set;}
		public Boolean hasProcess {get; set;}
		public Boolean hasStep {get; set;}
		public Boolean hasMyStep {get; set;}
		
		checkBoxEnv(Distributed_Item__c di) {
			distItem = di;
			approve = deny = false;
			process = [
				Select Id, TargetObjectId, isDeleted, Status,
					(
						Select Id, ProcessInstanceId, ActorId, Actor.Name, StepStatus, Comments 
						From StepsAndWorkItems
						Where 
							StepStatus = 'Pending'  and
							isDeleted = false 
						Order By 
							Createddate Desc
						Limit 1
					)
				From ProcessInstance
				Where
					isDeleted = false and
					TargetObjectId = :distItem.id and
					Status = 'Pending'
				Order By 
					Createddate Desc
				Limit 1
			];
			hasProcess = ! (process.isEmpty());
			hasStep = false;
			hasMyStep = false;
			if (hasProcess) {
				stepOrWorkItem = process[0].StepsAndWorkitems;
				hasStep = ! (stepOrWorkItem.isEmpty());	
				// Now, let's see if that step is for this user.
				hasMyStep = stepOrWorkItem[0].ActorId == UserInfo.getUserId(); 
			}
		}
	}

Load my outstanding approvals... so ItemsForApproval can be used in a pageBlockTable to display information about the object as well and the acceptance and rejection etc.

	private void loadApprovalItems() {
		if (itemsForApproval == null) {
			itemsForApproval = new List<checkBoxEnv>(); 
		} else {
			itemsForApproval.clear();
		}
		for (Distributed_Item__c di: [
			Select 
				Contact__c, Quantity__c, Status__c, isDeleted, Attribute_Value__c,
				Item__c, Item__r.Name, Item__r.Type__c, Item__r.User__c
			From Distributed_Item__c 
			Where 
				isDeleted = False and 
				Contact__c = :client.Id and
				( Status__c = 'Pending Approval by Distributor' or 
				  Status__c = 'Pending Approval by Manager')
		]) {
			itemsForApproval.add(new checkBoxEnv(di));
		}
	}

Go through them and do the approvals the user has requested...

	public pageReference doApprovals() {
		// Go through the Items for approval and process them accordingly.
		
		Approval.ProcessWorkitemRequest req;
		Approval.ProcessResult result;	
		for (checkBoxEnv env: itemsForApproval) {
			if (!env.approve && !env.deny) continue; // If the user didn't select, skip this.
			req = new Approval.ProcessWorkitemRequest();
			req.setNextApproverIds(null);  // No further approvals for this step needed.
			req.setWorkitemId(env.stepOrWorkItem[0].id);				
			if (env.approve) {
				req.setComments('Approving request.');
				req.setAction('Approve');
			} 
			if (env.deny) {
				req.setComments('Denying request.');
				req.setAction('Reject');
			}			
			result = Approval.process(req);
		}
	
		// Reload the Approval Items.
		loadApprovalItems();

		return null;
	}	

The portion of the page with the VF about Approvals...

	<apex:pageBlock title="Approve existing requests" mode="edit" id="approvals" rendered="{!areThereItemsToApprove}">
        <apex:pageBlockButtons location="top" >
        	<apex:commandButton action="{!doApprovals}" value="Do Approvals" rerender="approvals, distItemsRelList"/>
            <apex:commandButton action="{!doDone}" value="Done"/>
            <apex:commandButton action="{!doRefreshApprovalsDisplay}" value="Refresh Approvals Display" rerender="approvals"/>
        </apex:pageBlockButtons>
		
		<apex:pageBlockSection showHeader="true" title="Approve Distribution Requests" columns="1">
		 	<apex:pageBlockTable value="{!itemsForApproval}" var="app">
				<apex:column value="{!app.distItem.Item__c}" />
              	<apex:column value="{!app.distItem.Quantity__c}" />
              	<apex:column value="{!app.distItem.Attribute_Value__c}" />
              	
              	<apex:column headerValue="Comments">
              		<apex:outputField value="{!app.stepOrWorkItem[0].comments}" rendered="{!app.hasStep}"/>
              	</apex:column>
				
              	<apex:column >
                	<apex:facet name="header">
                		Approve<br/><input type="checkbox" onClick="toggleAll('ToggleApproved_1', this.checked)"/>
                	</apex:facet>
                	<apex:inputCheckbox id="ToggleApproved_1" value="{!app.approve}" rendered="{!app.hasMyStep}"/>
                </apex:column> 
                
              	<apex:column >
                  	<apex:facet name="header">
                       	Decline<br/><input type="checkbox" onClick="toggleAll('ToggleDeclined_1', this.checked)"/>
                	</apex:facet>
                    <apex:inputCheckbox id="ToggleDeclined_1" value="{!app.deny}" rendered="{!app.hasMyStep}"/>
				</apex:column> 
				
				<apex:column headerValue="Approval Pending?">
					<apex:outputText >{!if(app.hasStep,'Yes','No')}</apex:outputText>
				</apex:column>
				
				<apex:column headerValue="Waiting for?">
					<apex:outputText rendered="{!app.hasMyStep == false}">{!app.stepOrWorkItem[0].Actor.Name}</apex:outputText>
				</apex:column>
        	</apex:pageBlockTable>
		</apex:pageBlockSection>
	</apex:pageBlock>

Note that I'm not 100% certain this covers every case, etc.  but it did the portions that we needed.   And, I've extracted this out of a larger controller, so there may be some hanging references.   Either way, hope this helps to explain it... Best, Steve.